>I've got a set of holes as you describe in the rear engine tin on my '76 
>FI 2.0 L.  I haven't a clue what they're for (my guess was something to 
>do with A/C, which my bus doesn't have).  I hose-clamped several sheets 
>of tin foil over the holes to keep exhaust gases and road dust from 
>getting sucked back up into the engine.

My guess is that this is where the preheated air goes through to the air
filter. A thermostatic flap is located behind (FIF) the air filter, so
that air is either drawn directly from the engine compartment (cooler air)
or indirectly by having it detour through the preheater (hotter air). 

The preheater sits on top of the stock exhaust system, unless it has
rusted away. It has two openings, left and right. An aluminum hose
connects the left (resp. right) opening to the left (resp. right) hole in
the rear engine tin. Inside the engine compartment, a paper hose connects
the right hole in the tin to the air filter's thermostat. The left hole
stays bare. 

The preheater on my '75 is gone, so I have removed the paper hose that
goes from the right hole in the engine tin to the air filter and plugged
both left and right holes with duct tape. 

-- 
Sami ('75 Westf.)
dakhlia@wuecona.wustl.edu
http://wuecon.wustl.edu/~samid/