Heater blower motor only works

check the blower motor resistor

It's one of the relays in the system. I am not familiar with that system, but I think there are two. Check the schematic.

First turn the fan on low or med then tap on the heater motor w/ a screw driver, if it try's to turn on or does turn on its the brushes in the motor (thus you need a new one). Second you can try testing the switch w/ an ohm meter to see if the switch is burnt out. If this doesn't fix it then your into relays and such. Good Luck.