If I remember correctly, the process for the heater to start is that when the air temp goes below the thermostat set temperature, it connects the 2 wires together (hence the reason you can just connect the 2 wires together without the thermostat) which sends a signal to the heater telling it to start. After a few seconds (probably 5 on yours), the heater fan should then start. 30 seconds after that, the heater should light. Based on this assumption and the information you provided, it sounds to me like the wires to your thermostat are working properly and sending a signal to start the fan. When the fan goes to start up it sounds like it is drawing more current then is expected hence the reason your lights are dimming and the breaker is tripping. I would check the wiring to the fan and verify that it is wired correctly and make sure there are no loose wires floating around which could be making contact with another conductor and causing problems. You could also get a 12 volt battery and connect it directly to the fan to make sure the fan is operating correctly (turns on and spins). There is a slight chance that there is a problem with the fan motor and it is drawing more current then expected. It could still be a problem with the control board which might not be possible for you to diagnose.