Quote Originally Posted by daveS53 View Post
I bought a Haywire fuse/relay box for my car..... The headlight circuit does NOT use a relay, it just has a 30 amp fuse.
Dave, I don't believe that your statement is correct. Headlights are not fused, but instead run through a fast reset circuit breaker mounted in the fuse block. This is a safety feature, so that in the event of a short or over current situation your headlights will pulsate quickly vs simply going "black" if a fuse were to blow. I'm not sure when circuit breakers first appeared for headlights, but I know that it's at least fifty years ago.

Also, it is quite common to add a relay to the headlight circuit if high intensity, higher current lamps are used. The relay gives a shorter, higher gauge connection from the battery/charging circuit, allowing a more intense light without overloading existing headlight circuits. A relay is necessary if you opt for a HID (high intensity discharge) conversion. With the relay properly wired the circuit breaker still protects against a short, but the relay allows higher current to drive the lamps. I'm quite sure that your Haywire system has a small, rectangular headlight circuit breaker in the fuse block. You can Google/Bing "headlight circuit breaker" for a concise description of why this circuit is not fused, and you'll also find links to discussions of headlight relays.