It appears your blower motor is not working correctly on your 1980 Cadillac Coupe Deville. The vents are for both AC and heat. You may also be losing vacuum pressure. If you cannot find the source, your mechanic can help you do so.
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a diagram or a picture of where the heater core is in 1995 cadillac deville is
How do I replace the heater core in my 98 cadillac deville
97 cadillac deville heater controll
how do you change the heater core on a 99 cadillac deville? I need to know, "Kaiser."
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short circuit in the circuit
if its under the dash by your feet its the heater core is bad
comes out inside . remove glovebox and go in that way . about one hour .
If it is leaking coolant. And you changed the thermostat. I would then move to this section. Flush the radiator and the heater core. This can be done at a certified shop with a reliable mechanic.
Behind glovebox in heater housing...remove glovebox and picture will form....
Heater coil