In a message dated 2/9/2015 11:43:39 A.M. Eastern Standard Time,
jpalmer@MYMTS.NET writes:

What i  don't recall is how the switch works.  On my 85 it turns.  I don't
believe you can use it to preheat the cabin.  I think it only works with
the motor running (but i could be wrong - i know it sucks a lot of power so i
 wouldn't use it without the engine running.)  It will run  indefinitely.
It keeps running for a minute after the van is turned off  if you don't turn
it off first.
I seem to recall that on my 73 (or maybe my  69 notchback, also with gas
heater) the knob doesn't turn but is instead gets  pulled and is more of a
timer.  But my memory is  lousy.

the green knob says "temp" and has a plus and minus symbols on it.. it is a
 timer switch..the wbx vanagon appears to use the same timer switch as the
aircooled van.