Defogger Air Flow
My heater fan was very anemic. At first I thought the fan was bad, but after pulling the dash and doing some testing, I found that the air intake was poor. The normal air intake comes through a 2" tube from the 2 little grills on the front of the car. In my case the tube had been cut too long, and nearly touched the bottom of the plenum, so very little air could get through. I fixed that by cutting the tube a little shorter. (sorry, no pictures) After taking the heater off, it's just to the right inside the hole.
That improved the air flow quite a bit, but it still wasn't great. There's just too much drag forcing the air through a little 2" tube. So I made a change that lets me turn on air recirculation. (i.e. it draw air from the cabin.) Changes shown below.
The vent cover swung back over to cover the hole. The difference in air flow between open and closed is quite noticeable. When this cover is open, the fan has far less drag and blows noticeably more air. I've mounted a small handle to the cover, which will hang down just below the dash. This will let me reach down and open/close the vent as needed.