Replace heater motor resistor.
Remove the wiring harness from the back of your heater fan motor. Remove the heater fan motor retaining bolts. Remove the fan from the front of the heater motor. Reverse the process to install your new heater motor.
about 5 min.
Continental/VDO New A/C Heater Blower Motor PM4820
There was a recall on the windshield wiper motors. Check with your locate dealer and they may replace it for FREE!!!!
replace heater relay and or heater blower motor
It's right behind the glove box. However you must be sure that it is not he fuse or the blower motor resistor that has gone out on you first. You can test the blower motor with 12V power as that is what it gets on high speed. www.trailvoy.com Trailblazer enthusiasts forum.
Look at the link I posted, that should help you. (:
If it works but not on all speeds or is almost making the car stall at idle if you turn the a/c on at a light Than its probably the fan motor resistor pack its $25-$30 at autozone and real easy and fast to replace.
Replace the resister, if that doesn't work replace the blower motor.
if its a 1987-1994 unhook the coolant tank unplug heater motor and 5 or 6 bolt off heater motor and you are done to put on reverse process
Probably the blower motor speed resistor is partially burned out.