Leaking when it rains? Where in the hell is this water coming from?

That pinch weld, if it's the one I'm thinking of anyway, is where the upper cowl is joined to the firewall. It's also under the windshield. So, if you're getting water from there it's probably the windshield seal that's causing problems. You won't be able to do much of anything from the backside, even if you did the water would still sit in the windshield channel and rust that out. You may be able to sneak some sealant under the windshield gasket from the outside if you remove the trim piece and the gasket isn't rock hard. If the gasket is old and rock hard you'll probably have to pull the windshield and replace it.

A picture would help. The other possibility is that the cowl drains are plugged and the water is seeping in through the joint between the cowl and firewall on the side because water is sitting in the cowl, but a pinch weld sounds more like the windshield.