Why would it only get heat on the high switch for your car heater when it used to get it on all 4?

I had the same thing happen on a 2000 Stratus and it ended up being a bad heater blower control switch. I replaced the unit and everything worked great.

Answeri belive this would be a bad blower motor resistor check that because you car has two resistors for the heater one which controls low med and med hi and the other control only high for safety reasons basically so if you need to defrost your not left with out heat