I have an unusual problem. I have a bypass switch fitted to my cooling fans so I can switch them on and of as I wish. Strangely, recently when the engines running the fans wont work, when the engines off the fans work perfectly!!
I had this exact problem, the garage i took it to we're stumped as well so I've just got them wired so they come on with the ignition. Thought that always on was better than always off.