I have a 2008 Crown Vic with what I am sure is an air conditioning vent vacuum problem. The problem started recently after 40,000 miles of great service. When the air conditioning is on the air flows through the defroster instead of the front vents. When you let it idle for a while, the vents open slightly. Upon acceleration, the vents close. When you decelerate, they try to open again. I am sure I have a vacuum leak or a bad switch. A local mechanic told me he suspects a bad one-way valve. I am a decent amateur mechanic and would like to fix this myself. I looked under the hood and suspect the vacuum switches are located behind the dash. How can I access these lines and switches? Thanks!