Ben:
I was able to find the door contact switch used on eBay for a couple dollars.  Look for vw units that have a single retainer screw.  Most late 80/early 90 VWs used the same type.  There is already an existing location 1/2 way up the door jamb on the sliding door that allows for simple installation of the pin switch.  Run the connected wire down the pillar and up to the dash (or location of your alarm) and you're home free.  The added bonus is that you will gain the dome light function when you open the slider!!!

Brent

Brent Weide
Portland, Oregon
http://www.weidefamily.net/vanagon/