Any part on a GM or any other manufacturers car is their responsibility to ensure that it was installed properly. What if GM sub-contracted the wheels to be installed on the HHR and they flew off at 50mph and killed people, would that be GM's fault? Of course it would.

That would be like me reviewing someones medical file, writing a incorrect diagnosis and killing them then telling the family it was not my fault because I only typed the letters that appeared on the report paper, someone else manufactured the paper that the report was written on so it is their fault your Father died.

 

Maybe the installer(outside contractor) damaged the insulation during the install. Should GM remove the headliner of every sunroof installed in every vehicle? No, it's not practicle. Would "any" automaker(Toyota,Honda) do this? No. Is it possible that you accidently left your sunroof open when it rained and the insullation was damaged? Maybe. Would your sound system cause it to fall off? Maybe. Could it be that you just have one of those cars. Probably. But, don't shoot down a whole company because yours is a mess. Ours are fine.