first thing that comes to my mind is do you have a sun roof?
If so the gutter system is most likely leaking...

 

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I am happy to hear your dealer was able to find the source of the problem. As for pressurizing the cabin and using canned smoked, I suppose that is a possibility, however, I am unsure of anyone that does it. I would suggest you ask your dealer if they know anyone off hand. Many auto-glass places offer leak detection services. The more common causes for water entering the vehicle are clogged sunroof drains, door seals and clogged HVAC drains. These are only suggestions, not an official diagnosis. Again, I encourage you to speak with your dealer, they should be able to direct you to someone in your area.

I hope this information is helpful.

If you have any additional questions, please feel free to contact me.

Have a great day!

Louis
GM Customer Service