RHD harness on an LHD is like turning the engine backwards and wiring it with the stock harness.
I would start testing out both harness's. Then If I had to I would canabalize the RHD harness to make one working LHD harness.
A new "used" harness is going to cost you close to a grand.
You didn't see any rust??? Look in the spare tire well. Under the spare see if there's any rust there. Look all around the back hatch window gaskets and look for signs of leaking.
MK3 TSRM On-Line
Read through this to aid in your automotive selection.
You'll either have a good mechanic or you'll become one with this car.
These cars come stock with all kinds of issues.
I am willing to bet that the radio and janky wiring harness would be the extreme beginning of yours.
Lots of supras' in NJ. Welcome to the forum.