I posted a similar problem in the LS1Tech web forum, but I those guys didn't appear that interested in the problem.
Here is the last information I found out:
I contacted Pontiac about this issue after taking the car to the dealer to find "the side window defroster is working as designed".
I had an email response & talked directly to the Pontiac customer service rep. She indicated that they have no other owner complaints about the defrosters not working and when the dealer investigated the issue all the other cars worked in the same manner. She also said the only way GM will take care of the issue is if more complaints have been filed.
I can not accept the fact that a window vent installed on a car is not supppose to output air in the defrost mode. I've checked mine a couple of times, and when the defrost is engaged air is blowing out the side of the dash toward the connection point on the door (when the door is closed). But the air does not come out the door mounted vent. I think that leads to either a design or manufacturing flaw. Either way, I think it should be fixed.
Having the AC on or off does not change the amount of air coming out of the vents. So that is not an issue with this problem. The defroster system may work better with the AC engaged, that I do not know.
If everyone can join and raise the same complaint to Pontiac, care of
Customer Relationship Manager
Pontiac Customer Assistance Center
866-952-4368 ext. 58954 between the hours of 6 a.m. until 2 p.m. EST. Monday through Friday.
then maybe GM will fix the error.