E38 Headlight fuse problems

[plasticpig]

The headlight fuse keeps on blowing on my 7 series. I have had a auto electrician look at it today and he is suggesting that damp is getting into the circuit and causing a short and blowing the fuse. He couldnt detect where the short was though and an examination of the wiring dosent show any visible damage. Is this a likley cause or is it a cop out?