Are you speaking of the AC for a regular passenger Vanagon (86 and
later), a dash mounted AC (ala the air cooled Vanagons), or something
else?

In passenger Vanagon that I have, the AC has a housing in the rear,
with duct work that travels the center (spine) of the Vanagon up to the
front, and that is the order in which it would be installed, rear
first, and working your way forward. The ducts have bolts/screws that
attach to the frame members of the ceiling (through the interior
lining).

The front section also is attached with screws, 1 each on the sides and
2 in the center, as I recall.

Their is another type of AC I've seen for the Vanagons that had the sun
roof and it is mounted on the ceiling as well, but I don't know the
details on that.