I just built a home-made headlight relay harness for my '88 fox. I couldn't find any pre-made commercial harnesses that I could buy. I built it such that there is no splicing into the factory wiring at all.

I built it because after I put in halogen headlights, I could smell plastic burning and the plastic around the headlight switch was hot

Relaying the stock headlights makes a big difference in brightness too.

The finished product came out really good. You could hook it up to any Fox mustang in about 10 minutes.

Is there any market for producing and selling these? Just wondering