First
See pages 16 and 18 of the factory service manual attachment. By my reading:

Red = "OX2", transmitting signal from oxygen sensor to the computer; connects to the smallest engine computer connector (Connector B), pin 5

Brown = common sensor ground on engine computer (a.k.a. ECM)

Black/Red = when engine is running, 12 volts from EFI relay via circuit opening relay pin 5.

Red/white = "HT2" heater oxygen sensor 2, meaning the downstream oxygen sensor, connecting to the largest ECM connector (Connector A), pin 19

Second
I would physically locate the engine computer (ECM) on the Rav. The computer is well under where the radio is installed. As needed, remove the cover(s) well beneath the radio to expose the computer. At salvage yards, I have found the computer easily. Identify the computer's three connectors. This is also easy. My Haynes manual labels the computer's three connectors A, B, and C as shown:

Third
Look at the pinout diagram et cetera at www.rav4world.com/threads/1998-rav4-ecu-ecm-pinout.299318/#post-2704638

Fourth
Check the wire color on Connector A (largest connector) pin 19. Check the wire color on Connector B (smallest connector) pin 5. On the OP's Rav4, I am expecting, but do not guarantee, that Connector A pin 19 will be red with a white stripe and Connector B pin 5 will be grey with a red stripe.

Fifth
The Circuit Opening Relay is in the junction box near the driver's left knee. For a drawing of the location, go to Elle's 1996-00 Toyota Rav4 Site and see the guide "Relays and Fuses," roughly page 15. Find the circuit opening relay on your Rav. Use the drawing at the "Relays and Fuses" guide to identify pin 3 on the relay's connector on your Rav. What color is the wire connecting to pin 3? Per the attached wiring diagram on page 18, I am expecting it to be black with a red stripe.

Sixth
The only wire on the OP's downstream O2 sensor that has not been identified is the brown wire with a white stripe. I expect this is the computer ground. You can use pages 16 and 18 to trace the wire and see if in fact it goes where the wiring diagrams say it should go (behind the combination meter, a.k.a. instrument cluster). Note: Computer ground is not necessarily the same as chassis ground.

Notes
-- I did some re-wiring of the downstream oxygen sensor as a part of fixing a P0420 computer code. Unfortunately I did not take photos of the computer side of the wire harness.

-- Even with the battery disconnected, and without more study, I am not wild about taking a multimeter to any of the (possibly ECM) wires to do, say, continuity or voltage checks, especially on long runs of wire with multiple branches. I expect the engine computer to have capacitors that are charged. Also the computer wire harness is kind of difficult to disconnect. I would want to be cautious about the possibility of frying the computer.

-- Edit: A 1998 manual tranny FWD Rav at a salvage yard had a rear O2 sensor connector with the following wires:

Red/white wire, going to ECU Connector A, pin 19

Red wire with gray splotches every inch or so, going to ECU Conn B, pin 5. I did see a few possible junction points right under the radio where the wire color might change.

Black/red wire

Brown wire

 

Thanks for all the info! @Elle_Rav4

Got a chance to look everything over and trace wiring back to the ECU.

Connector "A" Pin 19 - Red with white stripe (HTR2)
Connector "B" Pin 5 - Solid red at ECU (OX2) Assuming this is my gray with white stripe
Circuit opening relay pin 3 - Black with red stripe (Has 12v while engine is running)
Brown wire does in fact head behind instrument cluster (Has continuity to ground as well)

Now the wiring portion

Black with red stripe (12v running from relay) should be the (+) to my O2 sensor signal
Gray with white stripe should be the (-) to my O2 sensor signal

Red with white stripe should be the heater (+)
Brown with white stripe should be the heater (-)

Am I correct in thinking this?

 

Thanks for all the info! @Elle_Rav4

Got a chance to look everything over and trace wiring back to the ECU.

Connector "A" Pin 19 - Red with white stripe (HTR2)

Please confirm: Are you saying that your Rav4 ECM Connector A Pin 19 has a red with white stripe wire installed?

Connector "B" Pin 5 - Solid red at ECU (OX2) Assuming this is my gray with white stripe

Are you saying that your Rav4 ECM Connector B Pin 5 has a solid red wire installed? Did you see any gray with white stripe wire on any of the connectors?

If it were my Rav, I would want to keep investigating. I might very well unplug Connector B and do a continuity test from Pin 5 to the end of the gray/white wire. (People do swap out ECUs pretty often and so unfasten the connectors. I just have not wrangled with the ECU connectors much and am probably being a wimp on the point.) Could this wire change from red (at the ECU) to gray with white stripe at some junction box? I am checking.

Circuit opening relay pin 3 - Black with red stripe (Has 12v while engine is running)

Are you saying that your Rav4 circuit opening relay pin3 has a black with red stripe wire installed?

Brown wire does in fact head behind instrument cluster (Has continuity to ground as well)

Great

Now the wiring portion

Black with red stripe (12v running from relay) should be the (+) to my O2 sensor signal
Gray with white stripe should be the (-) to my O2 sensor signal
Red with white stripe should be the heater (+)
Brown with white stripe should be the heater (-)

Am I correct in thinking this?

To me the point is to get the right wires connected to the correct terminals on the connector under the front passenger seat. You may be more up to speed on this part (which wire does what) than I am at the moment. I have to check my notes and the wiring diagrams. I have found there isn't really a universal coding system for the wire colors on oxygen sensor connections. I trust nothing until I have done a lot of study. Check back for edits.

 

@Elle_Rav4

Yeah, my Rav is as follows:

Connector A, Pin 19 is red with a white stripe
Connector B, Pin 5 is solid red and has continuity with gray with white stripe
Relay pin 3 is Black with red stripe
Brown heads to instrument cluster

And yes, end goal is to have the correct wires from the passenger side floor connected to the Bank 1 Sensor 2 O2 Sensor

 

Yeah, my Rav is as follows:

Connector A, Pin 19 is red with a white stripe
Connector B, Pin 5 is solid red and has continuity with gray with white stripe
Relay pin 3 is Black with red stripe
Brown heads to instrument cluster

Got it. I do not see a junction box in the wiring diagram. But the wiring diagram I attached above is likely specific to a certain country's Rav4.1.2 (1998-2000 Ravs). I hear you about the continuity but want to know what is going on with this red wire at the ECU (correct expected color, so far) that appears to become the gray with white strip wire at the oxygen sensor (ECU side of the sensor's connector). Did the prior owner maybe monkey with this wire in the past? A junction box, connector or similar must occur somewhere in the wiring to connect the red wire to the gray/white wire. I know tracking this down may be a miserable chore. I do not want the OP opening up carefully wrapped wire harnesses.

Regarding what wires on the sensor connect to the harness: Many Toyota wiring diagrams have drawings of the connectors and show the terminal numbers as well. These tend to be pretty consistent from one Toyota model to another. E.g. for a late 1990s Camry, from How To Test The Rear O2 Heater -P0141 (1997-2000 2.2L Camry) | Toyota 2.2L Index of Articles | Toyota Index of Articles :

The 2000 to 2003-ish Rav has the same rear oxygen sensor terminal setup as shown for the Camry above. What is described above for the Camry and a Rav4.2 matches well what is going on with the OP's Rav.

The two black wires on a Rav4.1.2 Denso oxygen sensor (Denso 234-4214 for my non-California Rav's rear oxygen sensor) are for the heater. The black wires should connect to connector terminals 1 and 2. The blue wire is for the Oxygen sensor signal and should connect to terminal 3. The white wire is for the oxygen sensor "ground" and should connect to terminal 4. The "ground" for the sensing element of the sensor is really just a lower voltage. More evidence, from the densoproducts.com site, for Denso 234-4214:

The photos from the Densoproducts.com site right above are consistent with the Camry drawing above and the connections the OP is finding (grey/white wire excepted).

Page 18 of the wiring diagrams I attached has the following terminal numbers (shown above) for the oxygen sensor connector:

• terminal 1 red/white in service manual and appears to be also red/white on OP's Rav = HT2 heater for oxygen sensor 2, meaning the downstream oxygen sensor, connecting to ECU Connector A, pin 19.
• terminal 2 black/red in service manual and appears to be also black/red on OP's Rav = when engine is running, 12 volts from EFI relay via circuit opening relay pin 5
• terminal 3 red in service manual (red at ECU on OP's Rav but then appears to maybe transition to gray/white by the end of the wiring harness), thought to be OX2, transmitting signal from oxygen sensor to the computer; connects to ECU Connector B, pin 5
• terminal 4 brown in service manual and appears to be brown/white on OP's Rav = common sensor ground on engine computer (a.k.a. ECM). Not really zero volts but rather, a lower voltage (like 3.0 volts?) than the oxygen sensor signal voltage.

 

@Elle_Rav4 UPDATE

Finally found time and did some digging into the whole "red wire" / gray with white stripe (terminal 3 on O2 sensor, ecu connector B, pin 5). By digging I mean literally digging under the carpet to see what I can find.

Gray/white runs to just under the radio then it gets lost in a much larger covered loom (As expected because it meets up with the remaining ECU wiring). I ended up stripping back some of the Gray/white white to find that its just an external coating over top of a solid red. This is in fact the connector B, pin 5 red wire in question.

I've attached a picture showing what I mean about the extra coating, ignore the bare connections - was using those for longer test leads for my meter.

I think we finally have this one figured out.

 

Wow, good work identifying that gray/white coating. I guess if a senior Toyota engineer said to me, "Which of the four wires most needs protection during normal operation of the Rav4?", then I would choose the oxygen sensor signal wire, from the sensor to Conn B, Pin 5. After warmup, the O2 sensor signal is critical to best operation of the engine.

I expect what "LetsGetBusy" found will help me and others somewhere down the road when we are puzzled about certain wire colors we find.

I was at a salvage yard the other day, mostly on a Rav4.1 fuel system part mission, but I also checked the salvage yard's 1998 Rav4's rear oxygen sensor harness. For the wire on which the OP and I have been focusing (ultimately connecting to ECU Conn B, Pin 5), I found a red wire with gray splotches every inch or so, from O2 sensor harness under the front passenger seat (and carpet) all the way to the ECU. I also found a junction connector (for the latter wire and other wires) right underneath the radio / above the ECU.

 

@Elle_Rav4

I'll be connecting everything later today to confirm the findings and hopefully get rid of my CEL. I sadly need to cut the connector off my O2 sensor to extend the harness to reach the wiring under the seat. When any vehicle ends up in a salvage yard here, cats and usually all O2 sensor are removed by the yard staff so finding another pigtail under the seat is going to be hard to come by.

 

LetsGetBusy, if you like things to stay kinda tidy electrical connector-wise, it might be worth checking some other salvage yard Toyota models for (both front and rear) O2 sensor pigtails with female and male connectors that you can use to extend the wiring as needed. At Elle's 1996-00 Toyota Rav4 Site, under the "Rear Oxygen Sensor Modification" guide, I explain how I used some salvage yard oxygen sensor connectors to make my electrical modification to the system look halfway decent. (Your Rav probably does not need the modification per se, so just ignore those details in the guide.)

At my favorite U-Pull-It Salvage yards, the cats are always gone, but the O2 sensors are usually still present.