I believe I can field this one. Think of a diode as a check valve for electrical current. It only allows it to flow in one direction. If this wasn't used every time he turned on the rock lights the reverse lights and the overhead lights would come on as well. Hope that helped.
I appreciate the explanation, I really do. But could someone let me know what size diode to use/ what type amd where to put it in the line?
Wakeride is correct, it prevents the power from flowing in one direction (towards the dome lights/reverse lights... we only want the current FROM them, not TO them)
Most common diodes would work fine and they are cheap. I think I used a 1N4001 ***CORRECTION, I USED 1N4006*** but a 1N4004 is also common and probably be better. there is a colored band(usually silver on a black diode) on one end of the diode that looks a bit like this: === (haha) the end with the band is the end that you want power to go TO, or in other words, the end with that band is the end that prevents power from returning. it's pretty simple, just splice that sucker between the 2 sources of power and wrap it up with some tape.