2005 Mercury Marquis Air goes to defrost area.

You are looking for a vacuum leak in this case. The vend doors that direct air flow are actuated by vacuum. When you hit the gas to accellerate, the engine vacuum drops. If there is a leak, the system can not maintain the air's flow. Thus, it changes direction.

Check under the dash for leaks and under the hood. If you are lucky, with the engine running, you will hear a hissing sound from where the leak is located.

Let me know if you have questions.

Joe