I have an old Magic Chef gas furnace in my garage. Gas is connected, but wiring for fan and thermostat are a question. 55K BTU, 12 amp blower. Need to know what wires to connect to thermostat. Also, there are three wires exiting furnace. I know which ones power the 110v blower, but there is a third (white) wire of smaller gauge. Don't know what this one is for. Can you help?