Chris Chubb wants to draw air from under the back seat, warm it, and blow
it back into the interior.  Chris, regarding getting benefit of blower and
engine fan, check out a Porsche 911's system (74-77 at least). They take
air pressurized by the engine fan, bring it through the electric blower 
whether the blower is on or off, then split it to send it to the two heat
exchangers.

Regarding drawing air from inside, my Vanagon has a fan that draws air from
under the back seat, and mixes it with air from the heat exchangers on its
way up front to be returned to the interior.  Is this on all aircooled Vgons
or just the ones with gas heater boosters?  Does anyone know what turns that
fan on and off?  I've never traced it.

B.