I have a 87' and it has a factory amp for each of the rear speakers as well. I think all 4runners had them from the factory. On the first generation 4runners the amp turn on (remote) wire is white. On an aftermarket stereo the remote wire is blue. Not to be cofused with Blue-White, if you use...
hook up all the power wires for the amp Black=ground, red=12v fused, blue=remote from stereo.
then use the RCA's for signal. hook up the RCA's for the front channels. Make sure to use the outgoing amplified speaker wires that corespond to the front channels you used. disregard the line level...
About a year and a half ago I was in the same position as you are now in. My motor had one foot in the grave and I was trying to decide what I should do about it. My wife wanted me to get rid of it and get something else. But I am am 4runner fan and knew that if I got rid of my runner I would...