David,
It just so happens that I wanted to get to my rear heater assy, last
weekend, and experiancedthe same BS, in getting the cover off the
heater.
Here is what I did, and it took all of 5 minutes.
I removed the front of the seatbase (4 screws) and took my die grinder,
with a small cutting bit, and cut the stupid piece that goes under the
vent, off.
BANG !!!
Off comes the cover!!
What a stupid idea the guy had that designed that gizmo.
Now I can remove the cover at will.
No more being a contortionist, or swearing.
A Dremel tool will work if you don't have any air, or a Die grinder.

Happy cutting,

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Terry 85 GL