Read the first thread on this issue
http://www.chevyavalanchefanclub.com/cafcna/index.php/topic,53909.0.html
and others:
http://www.chevyavalanchefanclub.com/cafcna/index.php/topic,81055.0.html
http://www.chevyavalanchefanclub.com/cafcna/index.php/topic,123835.0.html
http://www.chevyavalanchefanclub.com/cafcna/index.php/topic,142822.0.html
http://www.chevyavalanchefanclub.com/cafcna/index.php/topic,105666.0.html
http://www.chevyavalanchefanclub.com/cafcna/index.php/topic,150507.0.html
http://www.chevyavalanchefanclub.com/cafcna/index.php/topic,130658.0.html
and many more.
safelite sells the rear window fo OK price I think around $200 or so ...
GMs price is $$$$$
Junkyard
Or epoxy/