Looking in the Jim Osborne body assembly manual, sheet N1-200, view B shows a windshield seal assembly. This is NOT the rear window. It's shown as P/N D1ZB-65030A28-A1A or A2A. It gets sandwiched between the windshield and the gutter (?), and then a bead of sealant gets shot around it. I can't find the seal assembly, are they available? How'd you guys get your windshields installed?
I believe what you are looking for is this, 3M windo-weld windshield sealer. CJ Pony parts has it for $23.99.
Mustang Windshield Ribbon Sealer 1969-1973
Part Number: AD8612
Windshield Ribbon Sealer for 1969-1973 Mustangs.
1969-1973 Mustangs do not use a weatherstrip gasket for windshield installation and require this special sealer for proper installation. Measures 3/8" wide by 15' long.
I just took the windshield out of my 1973 Grande for a cowl panel replacement and that sealer stuff is 3/8" thick. This will be my first windshield replacement so I have yet to put it back in so I can't give you feedback on how it goes back in.
Michael