Cadillac Repair: Car Heater, fleetwood cadillac, vacuum line

Question
My car heater runs awesome while im driving but when I go to stop and im ideling for a bit its freezing cold I have a 1990 Fleetwood Cadillac, what could it be???

Answer
Could be a stuck OPEN thermostat, Fault in the AC/Heater programmer, Faulty vacuum line.

IS or HAS the CHECK ENGINE light ever illuminated while driving the vehicle and or has any other lights illuminated or any messages from the message display center ?.

What work or diagnostics of ANYKIND WHAT-SO-EVER have been performed on the vehicle trying to fix this problem ?.