Fog lights come on when door opened

Hi,
Strange electrical problem. 98 300 tdi rear fog lights do not work when dash switch pressed but come on when any door is opened even if dash switch removed!
Thought it was faulty MFU so replaced with new one from scrap yard but still the same.
Anyone have any ideas? could it be a faulty earth?

Cheers

most strange electrical problems are a result of a bad earth, you could always take a temporary wire straight from the battery earth to the fogs simply to test this theory?