why does my a/c and heat only work on the highest setting?

the blower motor resistor is burnt.take it out,you will see where it's burnt,replace it,check the connector also for maybe melted.there are a couple different ones for your car,so take the old one with you to match it up to correct one.4 cyl or 6 cyl and sedan or coupe,they will ask you this info at the parts house.

Possibly a switch problem?