Gently remove your side vents from the dash.

Then carefully disconnect the movement cam link.

Gently pry out the opposite pivot.

Then twisting sideways you can remove the vent stop.

Carefully remove the dead foam, you will need to make a template so you can tear it on one side only.

Now using a piece of foam thin ( I use old mother board foam I have a ton of it )

Place the dead foam on it and trace the outline be wide on the outside.

Cut it outside of the lines a bit
Now you have a new donut gasket.

Work the foam around the Stop into the groove.

Now trim it evenly around about ¼ of an inch

Insert the stop with the cam side first, and be careful… then pop the other side in to place.

Re-attach the cam arm…
Now admire your finished work.

Your side vents won’t whistle any more and the cold air will be totally cut off of your hands and won’t interfere with the side defrosters any more.
PS. You can do the center vents the same way..... enjoy.
To good Fixin.
Briano