Grand Cherokee AC air door issue

I own a 1994 Grand Cherokee and the AC air mixture door sticks sometimes
and will mix hot and cold air for a few moments, then I hear a
light thump and the door closes and cold air comes out the vents.

I think the linkage that controls the door is sticking and it is dirty
with debris around the linkage. Where is the linkage control located on
the Grand Cherokee and what does it look like ? The underside where the
floor vents is where the door is located, but I cannot se the linage like
I could on older autos.

I see a small bell shaped cylinder with a line going into it, but I'm not
sure what is is. The part in under the drivers side near the center where
vents branch out from main duct.
Any help would be appreciated.
Thanks.

WB