i guess the only thing to do is start pulling your dash apart and systematicly cover and plastic bits that float around (ie cable connectors) with foam sleeving and try to make sure as little as possible is moving. i don't know mk3s at all but cant be too hard. if its a vibration type of rattle it may be harder to track down as its normally the road and engine causing the vibration.
just don't stick your head in the dash while driving about trying to get it to rattle, sure ifre way of ending up in a crash!!! but you're far too sensible for that eh?
