Sounds like it could be the common resistor pack that has failed, have a search on here for resisitor oack and loads should come up, if you want to diagnose if its faulty,
Locate it under the drivers side scuttle panel, disconnect the connector going to it, big red/black one with VAG group stamped on it funnily, and bridge the connections 1 and 5 with a bit of wire, if your fans then come on full blast, you need a new or refurbished pack