In the course of time the ends of the cables that control the fresh air ventilation will weaken and break. The flaps that shut of the fresh air were engineered to be open, so that if the cables break, air will run thru the vent. This isn't so bad in warm climates, but here in Canada where the crisp chilling morning air it is a tad uncomfortable. Here is the description of the work I did to repair mine, as detailed as possible.

Fresh Air Controls Removal: Disconnect the battery ground strap. Remove the passenger front seat. Remove upper dash pad by removing four 10mm bolts under the dash. Don't worry f the outer ones pull away from the pad, they can easily be glued. Remove fresh air vent control knobs by loosening the set-screws in the ivory colored plastic knobs, with a 3mm screwdriver. Remove the chrome unit trim plate by pulling towards you. And lastly, remove the fresh air control unit by taking out the two 10mm bolts in the center of the dash. Loosen the cable clamps and screw barrels with a Phillips screwdriver and then disconnect cable sleeves and cable ends from the control unit.

Fresh Air Box & Flap Removal: Follow the steps above. Remove 10mm bolt holding glove box retaining strap. Remove the two screws for glove box striker plate and carefully take out the glovebox. Remove all dash gauges and connect wiring to the gauges and immediately identify them with tape and pen. Then using an 8mm wrench, loosen wiper arm locking bolts at the hinge and remove. Be sure to mark them "L" and "R" for reinstallation. Remove nuts for wiper shafts and take pieces out. Disconnect wiper motor power wires (3-connector) and mark wires. Remove the 10mm bolt holding the wiper motor to the body and remove wiper unit. Remove a 9mm bolt from each fresh air box at the topside of the dash, then from underside, remove the two 9mm bolts that hold the box the the cowl grills. Disconnect the drain pipes from each box and warm air supply to the center drain outlet. Now, starting with the passenger side box, gently pry the box away from the cowl and remove it.  The driver's side box is slightly more difficult but it is easier by disconnecting the washer hose from the jet and taking the box across towards the passenger side then down to the floor.

Fresh Air Flap Repair: Flap and cable end are concealed on each box by a black plastic plate, riveted at each end.  Between the plate and box is a foam rubber gasket the incorporated the flap.  The flap is reinforced by steel and the cable end is secured to the flap by two hooks that are part of the steel.  The flap can be accessed by carefully drilling out the rivets and peeling the foam rubber from the plate.  The flap can be replaced by a piece of black rubber with the original steel hook plate cemented to the surface.  The cable hardware is rarely damaged and can be reused.  The cable is identical to standard bicycle brake or gear cable, can can be cut to length.  Reuse the small rubber disk that goes between the hardware end and the hole in the box.

Finally: While all the above components are out of the car, it is an excellent time to restore and detail the items, like the plastic screens that are on top of the box (below the cowl grills) and fresh air control unit too.   All controls pivots, wiper pivots, nut threads and bolt threads should be lubricated where necessary.  All this hard work will have paid off for many years to coming using your newly restored fresh air vent flaps.

From Paul Colbert:

Re: obtaining black rubber to recreate the air flaps: I had to go the route of recreating the flaps as mine were rotted away.  None of my local stores had sheets of rubber to work with.  One good source for obtaining sheet rubber is through the McMaster-Carr Supply Company web site or their catalogue (562 692-5911).   This THICK catalogue has everything you can possibly think of in terms of hardware.  Order the catalogue first!  When you call to get it they will ask for a business name and address.  I gave them a company name and my home address. Once you get the catalogue measure the thickness of the rubber you need.   They'll have it in stock.

Re: replacing your fresh air vent cables:  I did take mine down to the local bicycle shop to look for replacements.  I did find a set but I had to grind down the cylinder at the end of the cable to fit into the metal strip hooks in the flap.

Re: replacing your plastic screens that are on top of the box (below the cowl grills)  part # 343 259 141:  These screens always seemed to get "junked up" over the years.  I've seen some cars where the owners didn't even bother to mask the area off when painting their cars.  You will be able to recreate these from items from McMaster-Carr.  In the catalogue they have metal (not plastic) screen with the exact number of holes per inch than the original screen.  I ordered the screen, cut it the appropriate size, and then had it zinc plated black.  (You can paint yours if you don't want to take that extra step).   As far as new rubber to mount the screen, it is also available in the appropriate thickness from the catalogue.

Re: restoring the fresh air box itself:  When it rains and your front floor is wet, most likely your boxes are rusted out or the "seam-sealer glue" seal around the box joints have leaks.  Have the boxes sand blasted, or dipped (I did the latter) to remove all paint, rust, glue, etc..  But before you have this done make a note of where the "seam-sealer glue" goes and how much to put on!  This glue can be obtained via mail order from the Eastwood Company.

After the boxes come back clean from the sand blaster, place the "seam-sealer glue" on the seams in the appropriate places.  After it dries fill up the boxes with water and check for leaks.  The boxes, with the glue on it, may now be painted.

Here is a shot of them freshly installed under the dash