Need help strange problem

[Focal]

I was working on this womens car at work. And the problem is that her passenger door speaker cuts in and out when she is listening to a cd. But when she has the radio on the speaker works normally. I could'nt think of why this would happen. I told her to come see me on saturday so i could try and fix it again. Hopefully i maybe able to get a clue from one of you guys.

[ariesdude]

Since its working fine with radio - bad wire connections are ruled out i guess.
you may have checked this already - but i will say it anyway - did you check if a different cd works fine .
If it does the same with all cds then the digital logic in the HU that reads the tracks or the sound chip that handles digital to analog conversion and or preamp may be bad. It might me as simple as dry solder joint or moisture from the ac vents getting in there and shorting it out (someone might have spilled coffee/soda in that general area too) or something like that. I would probably suggest replacing HU - it might even be covered under warranty if its oem and the car is young.