IF you remove the outer rear glass molding, you'll probably find a place near the lower corner that has become a new "water drain" over the years. GM's were very bad about that, starting in about '71. 3M came out with AlumaLead just for that fix!
The water will pool in the lower corners. If there's not a good coat of paint there, rust will happen over time. No place for the pooled water to drain to, as that's the lowest place on the sheet metal.
The rear glass should be held in with windshield urethane sealer. Like the front windshield. In the short term, you can clean out the residue under the molding area and put some of the same sealer over the cleaned-up "hole" area. Then put the molding back on and do a better fix later. Some POR-15, as others have used, sealer and body filler, might be a good choice. Best fix would be to remove the rear window glass, grind and clean everything up, then put the appropriate body filler in place, as the AlumaLead, refinish, and such.
Knowing about the issues with the GM back glasses, when we got our '72 Newport Royal, I got some of the black "strip caulk" and put it around the bottom and sides of the rear glass molding, ensuring that the water would drain over it. Has seemed to work as it's still there. Helped a little with the front windshield wind noise, too.
Keep us posted,
CBODY67