My Bently manual talks about a dealer kit to cure a hot footwell problem...
I can't get to it right now, but I believe the fix entailed altering
existing parts and installing a few new pieces to redirect radiator exhaust
down and away from the footwells of the car.  Maybe your vehicle either
needs this fix; had the fix, but not done right; or there's a hole in the
steel down there that ought to be sealed.  Maybe a boot or grommet that has
slipped?

Randy, Carla & Anna Huyck, Olympia, WA
'75 Rustfalia
'85 Vanagon GL
'90 Fox GL