I have a 2002 mercury grand marquis, and the air condition only blows through the defrost not the vent please help.

Check to make sure the engine vacuum is getting to the heater/ac control. Bad vacuum hose or leak. Many vehicles use engine vacuum to move heater doors and controls. When the vacuum is absent, it usually defaults to air blowing out the AC vents.

Mode door is stuck or broken .

the blend door in the lwr air vents is stuck it should be down by the squerrl cage also it could be electronically controlled

you are correct and its a night mair to do

If all air is coming out the defroster reguardless of the position of the selector, you have a vacuum leak or a vacuum line is disconnected.

Controls use vacume from the engine to operate and under exceleration , vacume is nill. so they got to nutral position.More anoying than anything else but normal in most cars.

manual or electronic heater/A/C controls?

Could be loose vacuum harness on back of control head. Need to put 10" tie straps on to hold it tight to control head.

check the doors in the heater ductin and make sure all the door motors r workin properly

The problem is with the blower door actuator. It's vacuum driven and controls the air flow from the defrosters, vents, and floor. With the AC on, open the hood and check the lines (very skinny rubber lines) at the firewall in front of the passenger and work back ward. You're looking/listening for a vacuum leak, either a pinhole or maybe a worn end in need of repair.

nope but ya can look in junk yard and save lots of mula