There is no 'control knob' ..
there is a lever that you can reach with your fingers, through a hole in
the rear heater cover.

if the cover is not in place...the poster could be looking at a
knob-like thing ..the bleed valve.

Bentley would not say much about this leverthat regulates coolant flow .
but the van's Owner's Manual sure does.

'Forward' on the lever is 'on'.
Aftward is off.

  I recommend having it at least mostly 'on' all year ..
as having no flow at all, ever, or for very long periods ,  contributes
to rear heater core leaking ..
a pretty common thing actually. .

Whether warm air comes out the rear heater is determined by the fan for
the rear heater being on or not, or course.

you always want at least some coolant flowing through the rear heater so
some fresh healthier coolant is always going through it ...the parts
last longer that way.  Old coolant is terribly corrosive.

Nobody on this list of course...
there are lots of vanagon owners that don't know about the heater
control lever under the back seat.
I was kinda shocked actually to find out VW did it that way.
I can't think of one other car that has a partially heater control under
the back seat.

On 11/18/2012 11:49 AM, Dave Mcneely wrote:
> Harry, that control allows coolant to circulate through the heater core.  Summer, off, winter, on.  To control air flow, use the knob on the dash.  mcneely
>
> ---- Harry Hoffman <hhoffman@IP-SOLUTIONS.NET> wrote:
>> Hi all,
> The heater under the rear seat has a control knob.
> Can someone tell me what it does? The Bentley only shows a picture but not how it functions.
>
>   Cheers,
> Harry
>
> --
> David McNeely
>