I absolutely love doing E Body side glass and this is one of the very reasons why I do...

...For I too, learned this lesson many moons ago...

The horizontal slider bar bolts to the glass with two different types of bolts. The forward one is a phillips head screw type bolt, and the rear one is a 7/16 head 1/4-20 bolt. There's also a rubber plug in the center of that bar that should be in place.

The front bolts screws into the plastic retainer in the glass and is a no brainer...It's the rear bolt that you MUST be careful with...It must pass through a metal spacer washer, a rubber hose insulator and the into the track guide on the backside of the glass.

If you don't have all these little items in place the glass will break...I'm 100% certain of it...