Beause there is probably a short/bad connection in the switch, or the blower motor itself
The compressor is coming on because you have the heater set to defrost. Take it off defrost and the compressor will not come on. I would suspect the compressor is locked up.
That would be fusible for older models
Is it blowing cold air only or not blowing any air at all.
it is a heater. heat. it blows heat. warm air
heater core???
Low coolant in the system. Defective heater control valve. Clogged heater core.
Check bulbs, fuses, and check for wiring problems
The fan motor has a short...change it.
check the switches and the motor
Are you blowing fuses or are the bulbs blowing? Blowing fuses would be a sign of any wires or metal contacts in your brake circuit grounding out or shorting on themselves. You'll have to check all of the wiring in your brake light/switch circuit to be sure.
It being an American vehicle
Your electric unit can blow fuses during the winter when you are running your heater for several reasons. The main reason would be that you have a faulty ground on your heater.