Heat on one side

sounds like the selector door actuator failed.

How is this fixed?

replace the actuator. utube it.

Look under the dash on passenger side for the vacuum actuator ... if your car has separate controls try this : with the engine running change the selector switch from defrost to any other position and you should hear the doors changing position ,,, the temp control only moves the mixer but on some cars with dual heat controls the diverter has to change before the mixer works ... almost all cars rely on vacuum to move the doors position so the engine must be running to check things out

Thank you.. I'll try this.. what if the vacuum system isn't working.. how's that fixed

Trust me ... you will know if the vacuum system isn't working and besides ,, you already said that 1 side of the heater was working

It does change when I go from defrost to other positions.. just no hot air on the drivers side

Opps .. I read wrong about what side failed so look under the dash on your side of the heater controls for a broken linkage ... change positions of both the direction and heat setting while looking under the dash (better yet use a mirror) that directional switch is both electrical and vacuum. I have found on some older cars the "O" rings either cracked or wore so much they failed to seal a vacuum port ,, if you don't have to dissemble the whole dash to check that then it may be the longshot fix ,,, Ebay has a slew of your controls relatively inexpensive

Thank you..