the Amp and the FM tuner are in the trunk. Most likely the plug they gave you is for a 1990 which will work, but the problem is, there is no place in the dash to hook it up at, it has to be plugged into the box in the trunk on the right quarter panel in order to by pass the OG system. Best thing to do is use multi conducter wire kinda like switch cable and make a jumper, then you simply take one end and connect the wiring harness to the jumpers and then the other end to the trunk mounted part of the stereo. This will bypass the OG stuff.
I know on Ford cars they make a harness to by pass the OG amp, but they are located under the passenger seat.
You might be easier getting the power hooked up, and running new speaker wires, the dash will be simple to run, the rear deck speakers wont be that hard to do.