I am surprised nobody replied to this before.
There is a known issue with condensation getting to the board behind the DVD overhead unit - the screen unit, but not the screen itself. The car is warm, the metal roof is cold, hence condenation above the roof lining.
From memory you remove it with 4 x 8mm socket nuts. Then CAREFULLY lower the unit down and unplug before removing. You will see some white powdery corrosion on the back of the PCB, in which case , yep that is the issue.
If it is not too bad, it can be brushed off but be careful not to damage any parts. If it is bad, it may need some light touch up soldering, or possibly some minor parts such as capacitors replacing, but if it is very bad you will need a replacement PCB and/or complete unit.
They seem to sell for about £70 or so on ebay, so not the end of the world.
I hope that helps.