opinion needed on a stupid pain-in-the-butt water leak...

I worked at a dealership. We were taught at school that the best way to find a water leak is to pressurize the cabin and spray soapy water (on the outside of the car) where you suspect your leak is.

Here's how to do it: if you have a working heater, turn it all the way up with the fan on high. (car running - don't forget to open the garage!) and shut the windows. Spray the soapy water around the windshield gasket or firewall or wherever you think the water might be coming from and check for active bubbling. Just like finding a leak in a tire, same principle. Costs nothing to try and only takes a minute or two.

This method works EVERY SINGLE TIME, never had it fail me. Windshield gaskets, door weatherstrip, roof seams, rear window seals on truck sliding windows - anything you can think of where water can get in.

Can't say I've tried it on an older car though. You may find more than you want to know about!