Thank you. Yes it was leaking into the blower housing. The wiper cowl is broken at the retaining screw. I vacuumed and flushed everything else, then sealed up the screw holes. Seems like it worked.
I did convert to double din. I used the Metra DP-3003 kit. The fit and finish isn’t quite as good as the OEM piece, but I think it looks cleaner than the kit that requires cutting the factory dash and putting the trim piece around it. As a bonus, the Atoto head unit was shallow enough that I didn’t have to trim the dashboard supports behind the radio.
Also, either SEM’s version of light neutral paint is significantly lighter the GM’s version or I miss read my RPO code.