strange electric window fault!

I'm having electric window nightmares on my 95 mk3 golf gl tdi 5dr - the drivers window will only work ( 80% of the time anyway! ) if the ignition is turned one notch but it won't work at all when engine is on. The motor and regulator are replacements - put in about 3 yrs ago and I have replaced the door switch but still no good.
It may be the wiring but then why does it often work ok if the engine is off - could it be a relay problem ?
ps the other windows work fine! ::confused
Cheers Jez