2003 Mercury Marquis heater won't switch to AC

Check to see if the blend door is broken from the actuator or it may be a bad actuator. However you hav eot remove the dash to get to it maybe, try taking glove box out first.

Broken teeth in the blend door actuator my friend! I've done hundreds of them. You'll have to loosen the dash by removing the srews on the top near the windshield and 2 bolts on each lower corner, lower the glove box and youll have a couple there as well and 4 nuts holding the column as well as taking the prndl cable loose so the column can go down, get yourself a 2x4 or like object and pry the passenger side of dash to give yourself some room and what you're looking for will be on top of the plenum, 4 8mm screws. The back one is pretty tight but it can be done that screw you can leave out when going back with it. Good luck!