i had this problem when a wire burnt out from the original stereo wiring, I sorted my stereo out, but then no lights or anythin on mfa screen, even though it lit up, If you look from drivers side up under dash next 2 centre console you will see a little black box with v.a.g on it an wires comin out of it, A wire from your lights switch goes 2 this and then onto original stereo so the stereo would dim when puttin lights on, Check this wire if you can find it, it is situated in haynes wirin manual, i ended up running a new wire from lights 2 this box, and it fixed it,