I have a lexan cover, too, and will make the same offer Inge is making to anyone else who wants it. But I like my vent and use it regularly. I don't have any leak issues now that I have the pipes, so I say, FIX IT!
I guess some people might put the lexan cover in during the winter to reduce the likelihood it might be accidentally opened. But I agree. It is nice to have, especially if you have a hard top. If you have a soft top, or no top, you don't need it. It is just extra venting over the heater.
the linkage may be gone or binding. it's the knob marked "fresh" and it's a pita to deal with cleaning up the linkage. i run softop in the summer and have early model vent doors so it didn't really serve a purpose to me.
I also have a '74 FJ40, and there is nothing inside the opening on the cowl except for the two holes that drain out through the engine compartment (as long as the holes aren't blocked by leaves!)
When the 'fresh' knob on the dash is pulled, it opens a door on the
passenger side that allows fresh air into the blower housing. You can look through the opening in the firewall at the front of the passenger footwell and see this working when you pull the knob.
I expect you'll see no air issue if you block the opening on the cowl. I did block mine - no change in heat, airflow, or unfortunately, the water that collects in the drivers footwell!
Mines the same as Desertdave's, no linkage to open it, doesn't appear to be physically capable of opening, and my fresh air knob does the same as D- Dave's. I guess its something that Toyota did on earlier models, then changed the fresh air vent but didn't want to change all their sheetmetal dies and just left the nonfunctional vent.
I am running a bikini top just now. I use the cowl vent all the time to blow the warm air out that collects just above the tranny hump, and it is truely helpful. Don't know if you can add the vent mechanism if it was never there, but it seems that if there is a hole, there had to be a way to close it. So there should be the mount holes, and therefore you should be able to find one used to put in.
...doesn't look to difficult. I'm going to dig in under my dash to see if there is anything there. On the outside it just has a cover tacked over the hole where yours has that opening panel. My slotted cover even mounts the same. It's too bad the PO cabbaged out my 69 so bad. There isn't much left in the creature comforts department.