I hope you paid by credit card. If the GM will not assist in giving you a refund, call the credit card company and protest the charges. The credit card company will put a hold on the charges and then give the dealership a chance to respond or correct the vehicle.

I know first hand finding squeaks and rattles is a pain in the backside, use to do this as a dealership tech back in the days when QC was crap. But you could find them if dedicated. Sometimes two techs would have to work together to find the source, but it could be found.

Keep to your guns. You spent big bucks, now it is time for the dealership to belly up to the bar.

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Experience: Where the test is given first and the lessons are learned later!