Heater will only work on high

Sounds like the blower motor resistor may be the reason for only working on high.
See if any voltage comes out of the wires of that resistor for low speed (tan wire), medium 1 (green/yellow wire) and medium 2 (light blue wire). Turn the key to run so the resistor has power.
As far as the only on defrost, start with check fuse #25 in the junction block, behind left side of dash.
Other than that you may have a faulty A/C heater control, but first check for voltage to the dark blue/white wires to all three of them actuators for the doors. With the control on heat and floor or face vents.