Rear window install problem

I've got a 67 mustang coupe. The rear window has been removed. I can't get the new window and gasket back in. I'm afraid I will break the glass if I push any harder. Someone is inside pulling the string fitted inside the gasket. Any suggestions? Thanks.

I've got a 67 mustang coupe. The rear window has been removed. I can't get the new window and gasket back in. I'm afraid I will break the glass if I push any harder. Someone is inside pulling the string fitted inside the gasket. Any suggestions? Thanks.

Might try spraying the gasket liberally with WD 40 to see if that would help.

soapy water works too, also check a criss-cross measurement to make sure the car doesn't have a bad repair. (replaced roof, or one of the sides)even check top to bottem, top sice to side, bottem side to side, also glass measurements,