RE: Access to Rear Heater
Very easy.  Prop open the rear seat.  There is a box covering the heater.
It's held in place with two or three screws that are very visible.  Remove
the screws and the cover comes right off.  You now have a couple of
electrical connectors and two hoses to remove, all obvious.  Then a couple
of screws to remove the whole heater/fan unit.  I've never disassembled the
heater/fan combo, I'll let someone else take over from here...