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.
If you do a Google search on Ford Explorer blend door there are lots of articles
A door panel on a 2004 Mercury Mountaineer is held on by several screws. Once all the screws are removed the panel can easily come off.
its your blend door failed or your blend door actuator failed. 1 of the 2 or both. cant find any info on tearing out dash in 03 mountaineer and it appears to me that my research says its in a different spot than other years.
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
the blend door actuater is bad
It is your blend door actuator. Good luck! It is a cheap part but an expensive time consuming fix.
Starting with the 2002 model , there is no automatic transmission fluid dipstick on the Mercury Mountaineer ( and the Ford Explorer , 4 door )
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"
How to take the door panel off a 2002 mercury mountaineer?
The 2004 Mercury Mountaineer does not have an automatic transmission fluid dipstick . I know that the 2002 and newer Ford Explorer four door versions do not have a dipstick for the automatic transmission fluid level .
On a 2001 Mercury Mountaineer : Underneath the Mountaineer , below the drivers door , mounted on the frame rail
They are located in the door latch on the door