I had this issue some years back with my 200, apparently it's a common fault with the C3 100/200 that there's too much voltage going to the headlight switch causing it to burn out the connector block (as was the case with mine) or the switch itself would fail. I purchased a kit online which had two relays and wiring which when installed took the load from the 12v going to the headlight switch. Now, this is a few years ago, possibly 7 years. if you have a look here:
http://audi100.net/Shop/shop.html and scroll to near the bottom of the page you'll see exactly the kit I'm talking about under 'Audi Lichtupdates'. I didn't buy from this site, I'm sure it was from ebay but you get an idea about what I'm on about. Unless any of our learned people here can say otherwise I think this is the solution to your problem, and I can safely say that I've never had a problem with the headlight switch since.