I would check my rad coolant level first. If that's okay then I would check the heat control cable. It connects to the valve that controls the flow of coolant to the heater. Its up under the dash above the gas pedal. If you follow it back a bit you will see its held by a little spring clip. The cable may have slipped a bit in the clip. Unclip it and put the heat to the hottest setting and then clip it back in. Just recently I had a similar problem with my '88. The air did not blow where it was supposed to because another cable had slipped in the clip