Eric,
VW installed two types of heaters for the Vanagon, the booster which was for
80-83 air- cooled Vanagons, and the auxiliary for water-cooled 84 - 91. It is
called the BA-6. 82-83 Diesel Vanagons had the DA-6, same as the BA but uses
Diesel.  The BA-6 will not fit in a Synchro.  The BA-6 was introduced in 1975
in the Type ll to '79. The Vanagon BA-6 to 83 is the same as the Type ll
BA-6. The 84-91 BA-6 auxiliary heater utilizes two fuel pumps, the booster
one.
 The BN-4 is a pre-75 heater that came as optional equipment in 1968 for Type
ll.  For the Synchro model there was a Canadian only optional heater booster
called the BBW 46 which heats the engine coolant. Something similar to this
is still available by Eberspacher called the BW4 Hydronic. They also make a
air heater as compact as the old BN-4 called the B1LC.This month I am
installing a BN -4 in a Synchro.  If you are interested I will let you know
how the installation goes. I also have several of the BN-4's in stock. The
most economical heater for the Synchro is the BN-4. The new Eberspacher
models are over 800.00.

Robert Keezer
1982 Westfalia w/ 95 Golf lll engine
Seattle