Well, its designed for a street rod, not a retrofit for a Mustang. It will have the engine harness, but the stock underhood harness ties into the lighting system and the dash harness. It won't be a drop-in and go, you'll need to get the lighting stuff out of your stock harness so that can stay, remove your engine harness, drop the painless in, wire up power and ground, and splice the painless kit into your dash where needed (A/C and gauges at least).