Alan,
You might want inspect the condition of the conduit near the left side of the truck lid/base of glass. That is what typically causes damage to the glass other than impact damage. When the cable is frayed or damaged, it gets caught between the lid and the glass as it is closed, cracking the glass.
More information regarding the type of rear glass can be found here:
http://forums.vwvortex.com/showthrea...1#post21624372
Damon


