They really are a simple system. Should be pretty easy to find the problem. You either do or don't have power to the line side of the relay and either do or don't have power to the coil+ side of the relay, and if yes, then is the thermoswitch closing when it should and providing a ground or not. If all is yes, then that leaves only the fan motor. Is there power to the fan motor when both the relay and thermoswitchs are closed and if so.. and the motor isn't running, is it's ground is good? If yes, then it's a bad motor.