The duct itself had a white foam seal gasket, sort of like double sided tape but not sticky. It normally remains fixed to the plastic when you remove it.

All I can think is that seal has died over time, or possibly if the "factory" air was not original to the car, whomever put it in may have only used 2 x screws (as the non A/C fresh air duct only uses 2 x screws) - it may need a new seal and an extra screw.

When you remove the duct put your hand up there and remove any dirt or leaf matter, and clean it all out properly. You may find the foam seal around where the duct joins the evaporator housing will fall apart too.

For the gasket- try Raven Products- they make draught foam tape in
various sizes & densiy.

Your local hardware store should stock it.

I have just done this recently to my WB the best way is remove the vent (I had the dash out for other reasons) I removed the vent completely because the foam gasket is old and dry and the foam seal going to the heater box will probably ,if it hasn't already will just fall apart. You will probably find that when you remove the vent that there are small rust holes in the plenum that are also allowing the water in. I used sicaflex on the vent appying a nice thick bead and to replace the foam at the heater box I went to an upholsterers buying some foam and cut out the foam to match the vent to heater box sticking it to the vent with sicaflex. The rust holes I just sealed up with sicaflex after I cleaned them out by poking around with a screw driver.This just a budget build to get the kid on the road ,He can do the proper rust repairs when he starts to earn his own coin. The draught foam tape I used on the rear of my dash vents to seal in the ducting tubes the tape size was 12mm wide x 1mm - 3mm gap. I also used this inside the vents to hold them in the desired position and to stop them flopping around. The vents are now as good as when they came from the factory.Note do not use clear silastic to seal stuff, apparently it causes rust, so I've been told. Cheers MS

3 x screws are correct for factory air fresh air duct and also for the driver's side blacnking plate. The non-A/C vents only use two of those 3 screws.That is pretty clean up under there, the electrostatic red lead primer got in there good.