Derale Fan Controller Problem

[flipper58]

My dual fans are not turning on. Twin fans on an RnD radiator with Derale fan controller. They have worked fine since installation until a few days ago. I have checked all wiring, including the ground, the inline fuse and every connection. I have bypassed the controller and thermostatic switch by connecting a direct ground to the ground wire coming out of each fan and both fans work with the ignition on. I tried jumping the thermostatic switch with a short wire to connect the circuit through the fan controller and no luck. By this process of elimination I have determined that it must be the switch. Am I wrong? Has anyone ever encountered this problem? I am no wiring guru, but I think I have diagnosed it the right way. Any help appreciated.
Phil