Here is the deal. I installed lights under the rear bumper, and one under each rear wheel well. Using relays, I have them tied into my back up lights on the truck. Here is the problem, if the gear shifter is put into reverse, and locked in, the lights do not come on. If you pull the shifter up a little bit, 1/4 of the way into park,the truck will still back up, and the reverse lights come on. Any ideas? This is the 2003 3500HD automatic, that I am working on.

 

The cable and switch assembly down at the trans needs moved slightly. Mine did that after I reb uilt my transmission. Loosen the small bolts that hold the switch and bell crank assembly that goes into the trans case and you can twist it slightly to make the lights come on like they did when the truck was new. Get a shop manual and look under the section on trans repairs and installation and it shows the switch assembly and how to adjust it.

 

Thank you, problem solved.