i have a 84 2m4. wiring is all messed up in back. i am getting ready to drop in a 3.4 with a carb conversion. the gauges will be aftermarket autometer type. push button start. and complete dash swap. i know i cant change just the rear harness. but what if i replaced the whole thing with one from an 85. or what is the best way to do it? as i cant find a complete 84 rear harness
It can be done, I don't recall if the forward harnesses are the same or not, but you would need to change the engine harness, rear lamp harness, and main chassis harness at a minimum. The door ajar switches and trunk switch were changed in '87(IIRC, mighg have been '86) to a switch mounted in the door/trunk latch, not the pin switch style. In '87 the gen 2 headlamp motors added, so the front lamp harness was redesigned then.
If you want to upgrade the harness to the later spec, it will be a WHOLE LOT of work. It would be less work to repair the bad harness you've already got, even if it entails replacing all the wire. If you decide to reharness the car it will be much simpler if you can grab all the wiring from a single donor, with the same options you've already got (AC, cruise, PW, PDL, manual or auto, etc.)