Yep, unless your windshield was pulled out, rust repaired and repainted a month ago, (in other words you KNOW it's rust free) it IS rusted at the windshield. They ALL rust there, I mean EVERY-SINGLE-ONE of them front and back glass. EVERY-SINGLE-GM product from early sixties through the late seventies rust there, EVERY-SINGLE-CAR, PERIOD.
Did I mention there my be rust in the window channel?
It can come from elsewhere of course, but the window and back channel ARE rusted. I remember doing a 68 Camaro with rust, windshield and back glass. I got them both cleaned up and repainted and the glass re-installed and there was still water on the passenger floorboards! It ended up being water coming down the antenna riding on the wire lead through a hole witout a rubber gromment to dripping off the wire onto the floor. LOL
I also had a 69 Nova that the trunk would get full of water off a broken gasket in a tail light! I would have never imagined so much water could make it's way through that tiny little gap in a gasket! But every drip of water that rolled down the trunk fell off it right onto the top of that tail light.
Now mind you, I am in sunny California, where I have had these cars with the original paint still on the frame, with the frame to body bolts still zinc coated and you can break them loose with a short 3/8 hand ratchet and those cars WILL have rust in the front and rear window channels.
I had a body shop for 13 years across the street from a glass shop where I got jobs on a regular basis, EVERY ONE ofthese cars window channel rust!
Another common place is from the heater box on the firewall. The seal from box to firewall, I have seen many leak there too. Cracks in the seam sealer on the firewall, yep, that too. However, NOTHING but NOTHING beats the amount of times I saw rust in the window channel, the others all added up don't even come close to rust in the window channels.
And the window MUST be pulled, you don't patch it up without pulling the glass and cleaning and sand blasting the channel and epoxy priming it. This is MOST EVERY TIME welding new channels in. At the very least a bunch of patches, removing the "nail head" like things that stick out of the side of the channel to hold the chrome moulding clips. Remove them and then use screw in clips.
I saw soooooooo many of these cars rusted in the channel, from super minor just started rusting to the complete gutter missing all the way around the glass! In all I saw I came to the conclusion that the failure occured when they painted the gutters at the factory and the "head" of that "nail" looking stud sticking out of the channel blocked the paint from hitting at the base of the "nail" like an umbrella shields you from getting hit with rain. The rust would always start at the base of that "nail" stud. That and the fact that the clips were metal and sliding them over the "nail" stud would scratch the paint leaving bare metal to rust!
Yeah, I have spent a little time doing that stuff. LOL
Thus that is the first place I would look.