[cmonref]

If you are getting warm air and it is coming thru the cabin heat system, and the heat lever (next to the hand brake handle) is fully down, then one or both of your heater control valves may not be completely closing. After they rust up, they bind in the slightly open position and some hot air from the heat exchangers gets thru and into the cabin. You may, or may not, be able to restore proper closing, or you might have to replace the heater control valve.