I went through something similar thinking mine wasn't working, constantly changing at night and day.
Before you start the truck, make sure the headlight is on auto, and double check what LewisB13 was talking about. Like a dimmer control knob...for the life of me I can't remember but pretty sure it's near the headlight switch and a tiny knob of sort.
Then, going to the radio and double check it's on auto. Start the truck...see if it works when the headlights turn on. It doesn't dimm immediately if you ever notice that the lights take a second to come on. I park my truck in the garage at night so when I start the truck, I remember it taking a second to auto-on the lights in which the radio dim simultaneously.
Something else to check is in the EVIC. I have a setting on my EVIC that if the wipers are turned on, the lights automatically come on which I then see my radio dim even during the daylight. I don't think the EVIC setting is a cause, but I'd check those things last. :4-dontknow:
Doesn't work after these suggestions, time to see Mr. Chrysler Dealer for some help as mine works as designed. :smileup: