Fan motor

I have a 1998 Buick Regal with 92,000 miles. The fan motor for the heater and air conditioning will only work on high. We have replaced the motor and the main switch and the relay behind the glove compartment still no luck but when my light sensor clicks on when in the shade and the headlights go on they dime the dash lights and then the fan will work on low speed. Once the lights go out the fan stops. Is there any conection in the headlight sensor or another relay that works both.