Mr Squirrel stated:

>> The ventilation box thing has funky seals and the air can't be shut off all the way, either, so that may be part of my trouble, too. However, removing the dash is the way to get to that thing. Removing the dash is also, I am told, a miserable job. <<

   Nearly 100% of  the unwanted outside air in my van comes through the foot well heater flaps.  It's easy to remove the duct cover in fronta the shifter and either stuff the ducts with whatever material floats yer boat, or tape over them with duct tape.  I left mine covered up for _years_ until I had to go to New York in March, but your climate may vary.

   Some folks have been known to tape the main air intake underneath the top grille...

     George/ATL
     '89 WesTiico, Wernher
      '67 Kombi, Puddles, with starter woes