Having trouble with the heater in a 90 Suburban. While driving heater switches back and forth from blowing warm air and cool air. If stopped at a light it will only blow cool air. Do not have rear heat (although wish we did) and just replaced the blower wheel (squirrel cage). Although it still doesn't seem to blow air (a/c or heater) as much as it should. Also, the vent closest to the driver, the one above the radio doesn't blow much air ~ almost none at all actually, but the one by the driver leg blows fine. Can anyone tell me what to look for? Thinking maybe the heater coil, but would that switch from heat to cool like ours is doing? And would it stop blowing warm air altogether when the vehicle is stopped? Also, notice I say warm air. Heater doesn't blow hot, only a luke warm. Thanks to anyone who can offer advice!