IAn,
     This rust is obviously closer to the front than the rear.  This
means it is likely
that the driver side panel or window is leaking water.  Now, if your van
is this rusty
this is not the only spot like this.  Look around carefully under your
van and examine
what you are getting into. You will find other areas that have holes.
    Closing the valve on the rear heater does no good to stop a leaking
core.
There is still pressure on both sides of the valve.

ian Butler wrote:

>A surprise at every corner as I strip down my syncro to clean it out!
>
>Over the past month or two I've been gradually pulling the interior of my
>van (seven-passenger) out, cleaning each bit of upholstery meticulously,
>washing long-forgotten surfaces, etc.  I also got a chance to look at my
>leaky rear heater.  I knew it had been leaking, of course, but kept it
>closed to prevent any serious puddles.
>
>Well!  Tonight I finally pulled the carpet out, and carefully dislodged all
>the pieces of sculpted squishymat to reveal: puddles, rust bubbles, and
>holes.  Ack!  To add insult to injury, the puddles are green -- the PO not
>only didn't ever fix the rear heater, but was using the Bad Kind of coolant.
>
>I managed to clean up the sandpapery, rusty undersides of the mats, and I'm
>going to thoroughly clean up the entire floor tomorrow.  (It's a bit smelly
>after sitting in stagnant antifreeze for untold years.)  I'm not entirely
>sure how to fix the holes and other rust damage, though.  It looks like I'm
>in for many long evenings with a Dremel and POR-15, and some sort of body
>filler.  Any recommendations?
>
>Most of the damage is pictured here:
>
>http://bluemoon.hplx.net/~ian/bus/rust/P6050154.JPG
>
>ian Butler / ian@bluemoon.hplx.net
>'88 Scirocco 16v, '87 Syncro GL
>
>
>
>