2004 Ford E-Series Van Location of AC Check Valve

Recently the AC in our van has been switching from the dash AC vents up to the defroster vents whenever the engine reaches a certain RPM. We are told that we have a vacuum system problem, and should probably replace the AC check valve. We bought one from the Ford dealership, but now our problem is this: where EXACTLY should we look for the existing check valve? And once we've found it, how do we get to it to replace it? Any guidance/comments/suggestions would be greatly appreciated.