A few weeks ago I picked up a sliding window for the drivers 
side rear (near the closet) off of ebay.de.

http://cgi.ebay.com/ws/eBayISAPI.dll?
ViewItem&category=15788&item=1780035039

I had never seen one before so I quickly snapped it up. My 
lament was that I did not have one for the passenger side.

Well, having done some surfing around it appears that they 
are for sale new. I saw "Volkswagen T3,type25 1979-1990 Rear 
slider o/s, n/s" windows for �94.25 at:

http://www.rainbowconversions.co.uk/windows.htm 

I have not contacted the vendor, but what else could it be...

Thanks,
Chris