The black plastic air vents on the rear-most windows of our '88 GL have what looks like a filter or a buffer of some sort inside.  I want to get at this to see if it is plugged and can be cleaned. What's the trick to opening this vent up?  I have tried just prying around the edges, but I fear breaking the plastic or the glass.  
The reason for my question is the pressure build up inside the van.  It appears that nothing is passing through this vent.  The doors won't close without a window open or a determined slam, and the heater blower labors unless a window is opened.  Is there a trick?

Gary

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