1997 C280, well maintained, 110K mi.

The center dash vent no longer flows air -- air only flows from the defrost and side vents.

When it was only an occasional problem, once in a while you'd hear a high pitched whistle coming from behind the dash. Also acceleration or deceleration could affect air flow from the center vent.

Research shows this to be a fairly common vacuum problem, but the fix gets vague. I though it's worth getting in to again here.

I've read that the problem can be caused by a faulty "PSE" air pump. Mine is located near the right rear brake light cluster.

Some people with this same air vent problem complain that their PSE pump is constantly on. Mine seems to come on and off as it should. However, this may be a clue: when pressing the trunk release button inside the car, the PSE pump runs, the trunk pops up and, after a few seconds, the pump stops. Problem is, the trunk will not close again for about 20 seconds. The latch mechanism does not release to allow the lock to engage. After a few attempts, the trunk will finally slam shut.

Could the inop center air vent and trunk latch be related? Does this point to a vacuum leak behind the dash?

Any help would save me hours and hours of troubleshooting.

Thanks in advance.