Reviving an old thread here but I recently discovered a feature that I think others may find interesting and FWIW water has accumulated on my '21 since day 1. It's just something I have learned to deal with.
While spraying mine down at a self wash stall the other day, the horn started beeping and the lights flashing just as they would if I had pressed the panic button. I immediately knew I hadn't pressed the panic button as it was inside my bag of chamois that were 10 feet away. That's when I noticed that I had unknowingly left the rear window down approximately an inch. So I guess there is some sort of sensor(s) in the cabin to alert blockheads like me in such instances.
Fortunately the alert occurred fairly quickly too as the cabin was somewhat wet but nothing that a chamois or two couldn't handle. Nice feature Chevy
Mavrik