A/C comes out where it wants to, the defroster, the floor and the vents.
 

I've got a 2007 Mercury Grand Marquis LS and when I have my climate control on automatic, and the a/c comes on, it comes out of the vents like it's supposed to. But while driving the a/c will come out where it wants to, the defroster and the floor. Eventually it'll come back out from the vents where it belongs.
Is this a vacuum leak or what?
Please help!

There is could be a vacuum leak within the plumbing or the control head on the dash.. Another possibilty, some vehicles have a storage container under the hood to prevent those HVAC issues during various engine conditions.
The system is designed to default to defrost with the absence of vacuum. Since yours also switches to floor, I would suspect a control head issue.

Is the control head what's on the dashboard that controls the heat and air conditioning?

Yes it is..

Thanks!

Issues with my 05 marquis
 

This guy is really good with fords and this video help me with my problem being stuck on defrost and floor vents. I replaced o rings and works like a champ now. Hope it helps
https://youtu.be/PtfucNa6R_o

Thank you very much!
 

I'd like to thank you so very much for posting that YouTube video on replacing the O rings in the head control unit. It worked! It solved my problem and saved me probably about $1000.00 if I had to bring it to a dealership they would have said I needed a brand new head unit.
Thanks again!

Your very welcome. Happy that worked and could save you/everybody some money.