Since your head unit used to dim properly it must have been hooked up correctly. Does your head unit dimming vary w/the dash lights or just dim to the same level at all times the park/headlights are on? Most Pioneers that I have installed w/ the dimming feature are fed off of the park lamp lead--not the dimmer lead. The wiring on the factory grey plug (starting w/ the side w/red wires) is: 1 battery feed, 2 switched ign, 3 dimmer (variable w/ dash lights), 4 park lamp lead (usually black/yellow--should be 12V or above as long as headlights or park lights are on), 5 RF- and 6 LF-. This is the same wiring ALL Mopars (exc. mitsu-mopars) have had since '85. If it is like the Pioneers I have worked with, the dimmer is fed off of the park lamp lead. If you have a volt meter, measure between ground and pin 4 of grey factory plug. If 12V or more there and the dimming on the head unit is not working the problem is in the head unit or adaptor harness to it. On some late model ('90-up) Mopars, this lead is controlled by the body control module and I have seen problems w/it not putting out enough voltage to turn this lead on. I think that the trucks still just run it off of the lead to the park lamps but not sure. If voltage is low or not existant on this wire w/lights on then it should be traced down. I would not recommend using a test light on it because if the lead is controlled by the BCM, it will pull too much current and probably blow that circuit out in the BCM (ouch!).
On the other hand, if it is hooked up to wire #3 (dimming on head unit varies w/dash lights), check that lead for varying voltage while turning dash light dimmer up and down. If no voltage there and the dash lights are working then the problem is with that wire.
Either lead 3 or 4 of the grey plug is compatible w/ all aftermarket head units w/a dimmer lead. In most cases the color of the wire for the aftermarket head unit and the wiring in the vehicle will NOT match (each car manufacturer uses different color wires for the same function--sometimes even from one year to the next) so don't worry about the wire colors matching--just about being connected to the correct wires
Hope this helps!