see if the fan works by bridging the connectors where it plugs into the radiator's thermostat, If that works, it may be the thermostat thats the problem. If the fan doesn't start when you bridge it, it could be that the fan's motor is buggered.
this is from experience of the probs on my mk2, the reason my fan didnt come on is because the radiator was so gummed up with crap that the thermostat thought it was cold still, when i flushed it all, it worked again.
anyway, you'll need to get more specific stuff from a mk4 owner...