Similar year models, the H6 and H4 door harness should be the same, except for the side view mirror deice button. If the 4-cylinder car has the side view deicer, then they should be the same. As far as where it connects to the harness at the body, you should be able to take the harness out of the six cylinder door and put it in the four-cylinder door and then it’ll plug up to the 6 cylinder car.
If Chuck wagon is a VDC model, the wiring for the stereo system is totally different than any other same generation car. The Macintosh system has power to the head unit with a din cable that runs down to the amplifier under the passenger front seat, and then the speaker harness connector is also under the passenger front seat where it would plug into the amplifier.
I’m assuming the car is a VDC with the Macintosh system because you said the big rear speaker.
When a Macintosh unit has a problem it will display a code on the screen. If it just quit working all of a sudden, you might want to check fuses.
