great advice, you all have been very helpful thanks again. I will test with a multimeter to be sure. I am on the hunt to find a 10+ amp that shuts off when the car ignition is off.I doubt if its easily visible, which side is live. If you dont have a multimeter, then maybe you have a light bulb with a wire soldered on to determine the live side.
You could also put the piggyback adapter in place with the lower fuse missing, there should be no power on your new red accessory supply wire if the adapter is in wrong way round.
My rav has 2 fuse boxes in the engine bay, on the passenger side fuse box there is a 20 AMP DEICER fuse. I am using this currently but surprised to find out that it is still powered even after the vehicle is off for 30+ minutes, is that normal? Is there an existing guide somewhere on this forum with which fuses are constant vs switched?