Heat comes out even with air conditioning on
Probably low on refrigerant. Have the A/C system checked out.
Check obvious stuff, like no or low refrigerant, and fuses. Same problem on my 1991 was the Temperature Blend Actuator, This controls the door in the plenum which in turn controls which is sent thru to the cabin.. A/c or Heat.Unfortunately requires a partial removal of the Dash ( Instrument panel.) Complicated even more with a passenger airbag.
Search Mercury Grand Marquis Questions
Mercury Grand Marquis Experts
Related Models For Sale