Heater in my 1992 GMC Sierra doesn't blow hot air.

Hi Nkra, Sorry to hear about your vehicle issues you are having. I did some research and did not find anything. Because I am not a technician, I would suggest that you take your truck into the dealer for a proper diagnosis. Best, Vanessa-GMC Customer Service

CHECK TO ENSURE ALL DUCT WORK IS CONNECTED OR VENT FLAPS R WORKING CORRECTLY, CHECK THE INLET TEMP OF THE CORE AND OUTLET TEMP, IF ONLY INLET SIDE IS HOT AND OTHER COOL YOUR HEATERCORE IS PROBABLY PLUG, IF THIS IS THE CASE REMOVE BOTH LINES AND CAREFULLY BLOW COMPRESSED AIR THROUGHT THE HEATER CORE I SAY CAREFULLY BECAUSE IF THERE IS A BLOCKAGE AND THE CORE IS WEAK YOU CAN OVERPRESSURIZE THE CORE AND CAUSE A LEAK. IF NONE OF THESE SOLUTIONS WORK, YOUR OPTION MAYBE A GARAGE OR DEALER, IT WILL BE EXPENSIVE BUT IF IT CORRECTS YOUR ISSUE THEN ITS MONEY WELL SPENT.

Check the a/c control unit