your best bet is to fix your current harness. which wiring harness is bad? engine harness, underhood harness, in-cab harness,harness going to rear?
they dont really make a new harness. there is a harness for Mustangs at Painless Wiring that alot of people use to change to a Mass Airflow EFI system. but thats probably way above what you will be wanting to do
Has the same issue with the engine wiring harness on the 88 5.0 that I bought last year. I found one on Ebay from an 89 working just fine. The only difference between the two was the 89 had a different connector from the engine harness to the fuel injector harness but the fuel injector harness was included.
What I found out during my research is that there is a difference between the 5.0 and 5.8 engine harness, the 5.8 does not have a knock sensor as the 5.0 does. i only paid 75 buck for the harness and he threw in some manifold bolts that i needed.