Not too sure what you have in mind, but it sounds like a heat pump. The
evaporator which normally removes heat from the vehicle interior would be
recircuited to reject heat. The condenser which normally rejects heat to
the outside air would in this case cool it. One problem is that heat pumps
lose efficiency with decreasing ambient temperature and typically need
supplemental electrical resistance heating below around 40 degrees F.
Another problem is the Clean Air Act which puts a crimp in the supply and
servicing of R-12 refrigerant, which is typically used in vehicle a/c
systems. And of course, the vehicle engine needs to be spinning the a/c
compressor to make the system work. 

If you just want to run hot water to the evaporator you would need to
compare its size with that of a heater core. Not too sure if the engine
coolant would be compatible with the evaporator core materials. An
after-market heater core for a Ford/etc. isn't that expensive. Or a
wrecking yard would have lots to choose from. 

Then there are controls to worry about.

John Pinkham
Portland, OR  '91 Camper