So far the only place I've seen this is at Spit Bits . Victoria British does not carry it. It's made by Wahler. There's been said to be a Volkswagen part that'll work too. It's easy to remove with some big pliers.
Some people have an override switch on their dash that turns on the fan on the fly.
I don't know the schematics for easily making one, but I'm guessing it is a switch between the connectors on the sensor. That's all I did for mine. A project box holding a simple switch rests next to my radio, and it works. I lost the picture for it. Testing the fan is easy.