It sounds like you are missing a ground connection. When the thremo-switch closes the electrics are finding ground through the fan circuit. Some of the earlier Betas (non LED lights) ran their lighting systems off the same ac that feeds the rectifier that feeds dc to the fan. Newer Betas run lights off the dc out of the rectifier. As stated, check for broken wires and good ground connections. Beta has done some creative wiring in the past that has led to these oddball symptoms.