Be Careful Out ThereWe often see mistakes made in windshield-molding installation. Windshield shops sometimes use mallets to install the stainless molding, a procedure that can chip paint and dent moldings. Frank suggests firm pressure on the molding using the heels of your hands. The objective is to pop the molding under the clip. You don't need a mallet, just firm pressure with your hand.
Sometimes, It Isn't The Windshield
Mustang body leaks aren't always rooted in the windshield and backlight. Cowl vents are the reason for leakage more times than windshields. Check your Mustang's cowl-vent dams for rust-through. Cracks in the lead seams at the roofline and A-pillars can cause a water leak. The same can be said for lead seams at the sail pillars and trunk area. Examine your Mustang's body thoroughly for leak sources. There are many, including rubber grommets around wiring harnesses, accelerator pedals, brake lines, and antenna leads. The rubber seal between the trunk lid and body will leak if exposed to substantial amounts of water. Quarter-panel end caps, taillights, side markers, and even trim pieces can be leak sources. Check them out.
Rear Window
Classic Mustang rear windows install the same way as windshields. Probably the most unforgiving rear window, also known as the backlight, is in the fastback. It leaks because it lies so flat, which makes it easy for moisture to collect around the perimeter and leak inside. That's why lots of sealer is needed around the fastback's rear window, especially around the outside between the rubber and body. Molding installation follows the same protocol: with a firm, soft hand.
1: Antique Auto Glass installs backlights the same way as windshields, with the rubber gasket going in first. Plenty of sealer is used between the gasket and body, and the glass and gasket. The base molding is then installed. The glass is seated in the base of the gasket first. | 
2: The rubber gasket's lip is popped over the outside of the glass as shown. This is a cleaner approach to glass installation. When rope is used, as Ford suggests, windshield sealer gets all over the place. | 
3: The rubber-gasket lip is popped around the glass perimeter as shown. |
4: Inside, the rubber gasket needs help. We pop the lip around the edges while someone presses from the outside. This way, everything seats into place. | 
5: Sealer is added between the glass and rubber gasket once the glass is seated. | 
6: Sealer is then applied between the rubber gasket and body to keep water away from the interior. |