Swapping wiring harnesses?

I have been chasing electrical problems in my 92' for a while now and have decided to get a new engine harness. My question what all do I need just for the engine, I know I will need the wires going to the TCU and all of the ones in the engine compartment. But are there any others that go back through the firewall I need for the engine to run. I haven't messed with XJ electrical stuff enough yet to know for sure where everything goes. So if anyone knows the bare minimum needed to make it run that would be great. I am not worried about lights or dash lights. I know I want the wires going to the gauge cluster but that is the only luxury item I really need or want.

Where are eco mike or 5-90 when I need them. If any body knows which parts of the harness I have to have that go back through firewall for the engine managment.

there are 3 connections through the firewall. one is the bulkhead connector by the brake booster. two is around accel pedal. three is for tcm on pass side just to the left of the heater box.

Is the one by the accelerator pedal the one that goes to the ignition switch? I had figured out I probably needed that one the one to the TCU and I had actually forgot about the bulkhead connector somehow.

why do you think that you need a new wiring harness?

I have a wiring problem I have been chasing for the last two months to no avail, and at this point it just seems easier to swap harnesses than keep trying to track it down. Here is a link to thread describing my problem if you have any input to add http://www.naxja.org/forum/showthread.php?t=963997 but I think I am going to go swipe a harness out of the only 92' in the salvage yards around me tuesday. I guess if it has problems hopefully I can locate them, if not I will probably just part the Jeep out.