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On right side of firewall there is a 'drum' with rubber plug on front. Pop out the plug, and using suitable tool (sorry, can't remember, screw driver, socket extension) push thru hole. You'll be pushing against surface made to be pushed, shouldn't hurt anything.
Remove the screen and pull it up by hand? This is how I got my 72' frozen wiper door unfrozen......flipped it up and down a thousand times with spray on the pivots......it freed up and works as intended now.
If you are doing bodywork then you need to remove the metal vent for refinish anyway right?
Yes agreed, I need to get the vent and cover prepped for paint but all screws are rounded off and I haven't hit an easy job on this car yet lol.... WD40 and elbow grease....
Charged - you will note that only one of the above explanations bears any semblance to the procedure written in the Shop Manual to accomplish your desired result.