This what we mean by loose...
This what we mean by loose glass in the doors of a '69 Mustang. The best way to solve the problem is to do exactly what Ford did for 1970: replace the glue-in glass with '70-type bolt-in glass. We had to improvise along the way, but once we had all the parts, the conversion on both doors took about a day.
We try to cover a wide spectrum of the vintage and late-model Mustang hobby in our how-to articles. In this case, we have a special treat for those of you who own a classic '69 Mustang.
The '69s (and a few early '70 cars) are the only years of Mustang that Ford glued the door glass into the mounting plates attached to the window regulator. While the '69 Mustang is unique in many appealing ways (for one thing, it's the only vintage Mustang with four headlights), it's also unique in this not-so-great way. Our experience with a '69 SportsRoof was irritating. We tried gluing in the glass, only to have it come out again after a few months. Furthermore, having the glass flopping around in both doors quickly became annoying.
The problem was solved by the folks at Muscle City Glass with reproduction door glass for '70 Mustangs. However, the glass is only part of the picture. You also need '70 plates to attach the glass and regulators together, which we procured from Mustangs Etc.
Finally, we called on Scott Drake Mustang Parts for a set of '70 window regulators. Although they look nearly identical to the '69 versions, there is a slight difference between the two; but you may be able to make the '69s work.
There are many other details you need to know to complete this swap, and we've done much of the R&D work for you. The accompanying photos and captions explain the particulars.
What Glass For What Body Style?
There are two shapes of door glass for '69-'70 Mustangs, as there are with the '65-'68 cars. Coupes and convertibles share the same shape, while fastbacks have a unique shape. Muscle City Glass carries both.

This is what you need for...

This is what you need for the passenger-side conversion. We did our research by performing the driver-side conversion first to determine what was needed. Then we did the photo sequence on the passenger side. The glass is available from Muscle City Glass; the regulators are available from any parts house that stocks Scott Drake Mustang parts. We don't know of anyone currently reproducing the '70 plates between the glass and regulator. Look for them at a shop that carries used parts such as Mustangs Etc.

To begin, the door panels...

To begin, the door panels need to come off. Remove the window crank and armrest/door pull as shown.

If you have nice panels on...

If you have nice panels on your car, be careful while removing them because the metal clips are stronger than the cardboard backing on the panel. Pulling on the panel will ruin it, and the clips will simply stay in the door. As shown here, try to get the screwdriver behind the clip and pry the clip out of the door to avoid putting any pressure on the cardboard. We had pretty good luck. Out of 20 clips (10 on each door), we ripped only one mounting hole during removal.

With all the clips loose,...

With all the clips loose, the door panel comes off the door.

Once you have access inside...

Once you have access inside the door, remove the glue-in window plates and regulator. Since the glass was already removed (because the glue wasn't holding), the plates were sitting in the bottom of the door. The rear plate comes out through the largest hole in the door. Here you can see where the glass glues into the groove at the top of the '69 plate.

Next, unbolt the arm of the...

Next, unbolt the arm of the regulator from the front plate.

There are five bolts that...

There are five bolts that attach the regulator to the door: four toward the front of the door and one near the back that attaches to one of the regulator arms.

Remove these five bolts and...

Remove these five bolts and the regulator can be removed through the largest hole in the door.

Shown for comparison are the...

Shown for comparison are the plates for '69 (top) and '70 Mustangs. As you can see, they are completely different. The '69 plates are glued to the glass, while the '70 plates are secured by three bolts each. Both attach to the window regulator in a similar fashion.