I have experienced recently a number of failures of the fan thermoswitch even though I had it replaced with the correct new part every time. When I got bored of replacing the buggers I just hardwired the contacts on so the fans run all the time when the engine is on (this is a 205GTI we're talking about).
And then one day, I grabbed one of the failed switches and opened it on a lathe carefully turning off the bronze lip that holds the plastic end on the sensor head. Inside, I found signs of water ingress and the contacts were corroded. I looked at everything and found there is a rubber seal running at the base of the plastic bit, under the lip. This seal seems to have failed and allowed water to creep inside, invade the switch and do its nefarious business on the contacts. Everything else was fine. Stupid crappy sender units and their retarted seals and lips. Made in Spain, probably during the siesta. I suppose next time I'll buy one of these I'll run a bead of RTV silicone around the lip just to make sure.
Anyway, hope this might help someone.