When I had a factory radio in this vehicle, with the radio on, my interior dome lights would come on and stay on until the radio was turned off. When I would ajust the volume up/down, the dome lights fluctuates dim/bright. three factory radios and now a after market radio later, problem persists. Can anyone help me?

It would help to know the year and model of the truck.

96 Ram 1500 4X4

I would start by checking the ground from the cab to the engine.

It sounds like the power switch for the radio is acting like a door jamb switch should, completing the dome light circuit and illuminating the dome lamp.

You need to trace the wires from the radio harness to the fuse box, and past that if necessary. Sounds like the radio power wire is tapped into the dome light circuit somewhere.