Jordan,
I would guess ya got corrosion,First I would spray all the linkage pivot points with PB Blaster an let em soak.
You may have to pry the thing open as the gasket may be rotted and along with rust has glued itself shut, you might want to get a helper to run the lever in the cab while you gently work around the perimeter of the vent on the outside cowl with a flat tip screwdriver till it pops open.
Once ya get it to pop open you will see what you are dealin with under there, I would bet you will need to remove it and clean all the years of crap outta the channel it seats in.
When I first tried to open mine when I got the truck it was like you stated it tried to open but wouldnt,I found the PO had siliconed it shut the gasket was gone and must have been leaking so they just glued it shut.
Hope this helps
Tim

1952 Chevy Shortbed

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If It Aint Broke Fix It Till It Is!