The Grand Marquis uses the same HVAC system as the Ford Taurus and Mercury
Sable and all three automobiles suffer from the same blend door failures. The actuator is located on the front side of the plenum box and you can see it if you fully open the glove box and look to the left over the transmission tunnel. The recommended dealer fix is to remove the dash and plenum box and replace the plastic door. There is a kit on Ebay from HeaterTreater that offers a cheap easier fix for this problem.
There are 2 common questions relating to the temperature blend door on a Taurus / Sable:What moves themHow to replace themThe "Related Questions" below address both
This sounds like a temperature blend door problem - with the door stuck in the heat position. See "Related Questions" below for information on Taurus/Sable blend door actuator motors and blend doors
If you do a Google search on Ford Explorer blend door there are lots of articles
The blend door is jammed
blend door is jammed.
the blend door is jammed.
I believe you are looking for blend door actuator???
With most heaters in vehicles the blend door mixes hot and cold air together to give you the temperature you want. Thus the name blend door
Its the blend door. It happened on my 98 Mercury Mountaineer. The motor shaft and the door have separated there is an easy fix look up blend door fix for 98 Mercury Mountaineer it just needs a small finishing nail and done
It could be the temperature blend door actuator.
Mercury Mountaineers have a "blend" door in the heater box. It is probably malfunctioning. You can find more info by searching for Ford + "blend door" or "no Heat"
1) Not enough refrigerant, 134-A 2) The temperature blend door is stuck on hot - See "Related Questions" below