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
I'm not sure if Mercury followed Ford with the introduction of the new HVAC system in 2003. That is probably the case and the system and problems will be the same as the Ford Explorer. Pictures to identify which system you have and diagnostic processes are available on heatertreater.net.
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It is your blend door actuator. Good luck! It is a cheap part but an expensive time consuming fix.
the blend door is operated by the blend door actuator motor which is electric.Fairly easy fix too...See "Related Questions" below for more
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.
Probably a faulty blend door actuator. Remove the glove box to gain access to the actuator. Replace the actuator with a new one.
spray your door latches with wd40.
To DIY you're going to have to pull the dash. If you're going to take it to shop, be prepared to pay for 6+ hours of labor, quite expensive. No NO NO do not pull the dash. On my 1998 it was outside the blend box. It's bad because of bad soldering in the motor circuit board, easy fix once you pull it. Yes the shops want 6hrs labor. My blend door shaft broke and I had to replace it. I found the bad soldering problem after fixing the door. Blenddoor.com has a fix for that door problem...........
The door lock, on your 1999 Mercury cougar, can be replaced by removing the inside door panel. The locking mechanism is easily visible and can be changed.
Are you really sure it is a '98 ?