I have an interesting issue and maybe I am just over thinking this. I installed an after market stereo in my 2004 F250. It works great except the dimmer functionality. The illumination wire (lt blue / red) from the truck went to the illumination wire on the new radio but the dimmer functionality is reversed. When i dim my interior lights / cluster, the radio gets brighter. When I brighten my interior lights it gets dimmer. I can bypass this by turning off headlight detection and just manually diming on the radio. I figured I would try here and see if any of you ran into a similar situation. Any help with this or suggestions would be greatly appreciated.

 

Just out of curiosity, how is your radio grounded ? I wouldn't go by hoping it is grounded through it's mounting.

Usually they have their own ground wire, and did you connect that to a known GOOD ground?

 

The new head unit had a single black ground wire, and that is connected to the black & black and lt green ground wires the factory radio used. Since the after market unit is functioning except the dimming being inverted I assumed it was a good ground, but that definitely gives me something to investigate. For what it is worth i attached the schematics to show you. I spliced the grounds together because i wasnt sure which to use on my one new ground. That may have been a mistake.

The OEM unit had 3 grounds.

The bottom left part of the schematic shows the wire controlling the dimming function.

New unit wiring diagram.

 

From a brief look at the diagrams, I suspect you are getting some "feedback" into the interior lighting system.

I believe you are on the right track if you provide "separate " grounding of each system.
Please let us know if separate grounds corrected that alternate path, thanks.