George Golf wrote,

" If I had to chose only one, I'd take the rear heater.  Remember, the only
  thing the front heater is heating is outside air."

You can reduce and/or almost eliminate fresh air through the front heater
plenum by covering the outside air intake screen.  I use cardboard and duct
tape.  This keeps cold air out in winter, and humid air out in summer (when
you may wish to use the a/c).

Budd Premack
86 Syncro, 88 Wolfsburg
land of Sky-Blue Waters
Minneapolis, MN