GM Headlight Switch Circuit Functions | American Autowire GM Headlight Switch Circuit Functions. GM Headlight Switch Circuit Descriptions: 0. Ground output wire through the rheostat. 1. 12 volt battery unfused feed into H/L switch circuit breaker I have your Factory Fit engine harness wiring, how do I hook up the wires to my starter motor? I have three posts, two small posts and one large post on Wiring Headlight Relays - Rowand Wiring diagram. The wiring diagram below shows what you need to end up wiring to make this work, so if you know how to read a wiring diagram and feel like "skipping ahead", just go click on the thumbnail for the wiring diagram and check it out in full size, full color glory.