So not sure if posting correctly, but as recent 06 owner it seems water is leaking in from 2 sources: the cowl under wipers that needs a butyl patch & after heavy AC usage. I def 1st noticed in late summer, but all seems fine now as live in MN & no more using AC or any rain as it's been below freezing last 2-3 weeks. Also concerned if this has caused excessive rusting due to previous owners never fixing it?

 

The AC runs in defrost.

 

It may not be raining, but snow melts so you can have the same issue in the winter. The leaking butyl patch NEEDS to be fixed asap to prevent damage to the fans resistor as well as the carpet, floorpan etc. etc...

Re: rusting? too many variables to say yes or no. You have to take the cowl off and eyeball it from on top, as well as from the bottom.

 

I might add that below that patch is a drain that can get clogged. I know this because mine did as that area was filled with 13 years of mesquite leaves.
Just look in my photos
It doesn't take much to remove the cowl repair the patch and clear the drain

 

I finally had time to investigate the source of my rain leak today. It's now all cleaned out. I'm just waiting on the butyl patch to arrive from the dealer so i can put it on tomorrow.

 

And here’s a video with the new GM replacement patch install