my first thought is to eliminate the obvious. Check the speaker(s) make sure they are functioning. Once you have eliminated that, then u should hear something and that always make one feel better when trying to fix a radio issue
Next, where is the ground? Is it a good ground? Sounds... like a grounding out issue on the dimming, or too much draw on a specific circuit. I would test the radio out of the truck, test the speaker with a working unit, then if both work turn your attention to the wiring. If all else fails, rewire your own power and grounds.