We finally got a bunch of rain here in CA. Drought relief! Yay!

Wet driver's side carpet! Boo!

I suspect this is the culprit, although missing/popped rivets at the front could also be it, as the carpet is getting wet at the front:



I have Dicor non-self-leveling lap sealant in grey. I figure no one is going to see this unless they're on the roof, so I won't worry about the color. Is this a recommended material for this kind of problem?

That should work; clean the area thoroughly with a final wine of acetone. Another solution could be epoxy.