I'm in the process of updating my interior to the 01-04 style, and I just got a dash from an 02 to swap in. It came with all the wiring harnesses and everything, so does this mean that I can put in a gage cluster from an 01-04, or do I still need to use my existing 95 cluster?
As far as I know, you'll still need the 94-95 harness and cluster. The cluster in the 96+ cars is pinned differently than ours, but I don't know about the harness. The 99+ cars used a digital odometer and the EEC in our cars don't have that function.
Use your original harness. It is needed to operate the rest of the car. You are going to run into an issue with the radio though. 94-00 Mustang dashes are the same, just colors differ. 01-04 Mustang dashboards have the larger opening for the updated single din radio. The plugs are different on those radios too. Good luck with that situation. You will just have to custom wire the 2001+ radio to the 95 harness. I hope the 95 you have does not have Mach 460 to ease this radio deal.
I already have an aftermarket stereo installed, so there should be no radio issue once I get a dash kit to make it fit in the double-din sized hole. So I should use the entire wiring harness from my current dash when putting in the new one?