I recently bought a used 2003 Silverado. The heater only works when both driver and passenger sides are set to 90 degrees. anything less it gets cold no matter what. Any ideas??

 

hvac control module prolly. or heater door malfunction

 

If when it DOES blow hot, set at 90 degrees and you get good air flow at that setting then I would go with the control module. The heater door should really not matter to where the temp is set at. (I don't believe anyway, but I could be wrong.)
Does the air flow change when you adjust the fan speed at all? I mean does the fan work at all speeds, or just on high perhaps? If it works on all speeds then again I would guess the control module as sfarrell05 suggested.
Good luck to you.