Welcome to the forums alphaville.
What year is your wife's RAV4? Most rear wipers are designed the same, so this info. should work. You need to lift up the cover on the end of the wiper (near the bottom). Under that you will see a nut. Unscrew the nut and remove. The rear wiper is press fitted onto a spline. You need to wrestle with that arm back and forth until it comes off of the spline. It is probably frozen on so be patient, maybe some oil/lubricant will help.
Once you remove the wiper, note where the location of the wipers last position. Re-position the new wiper and press fit back onto the spline. When tightening down the nut, it should press it back all the way down.
There are many online Toyota parts sellers, but I don't know any which are reliable. I usually buy most OEM Toyota parts from the dealer.
By the way, a modification that I have done is using a Toyota Highlander (any year) rear wiper. They are virtually the same size and look much cleaner in appearance.