So I cleaned & rebuilt the windows and then my husband helped me with dremmeling and drilling and punching out the screws that broke or I cut. This took a lot of effort - basically you should do anything you can to get these screws by unscrewing out to avoid this.
Finally the window was ready for reinsertion. The window hole was prepared by reglueing the carpetting (I pulled it off to clean the edges well and to install new marker lights
which are just above the rear window) with 3M Super 77 spray glue, then cleaning the edges on each side of the fiberglass window hole with Goo Gone then with Acetone.
I apologize for no photos of the install but we were racing against the sunset to get it done, and trust me I was not about to wait another day.
First we dryfitted the window to get a sense of how to get it in - it stuck in a couple of spots and it is better to figure how to get the window in now, rather than once it is sticky with butyl. There were a couple of areas on the fiberglass frame (upper and lower midline) that were depressed inwards - so I added a little butyl at these spots to flatten out the area where I would apply the window. - You can see this in the picture of the open window hole.
We applied the butyl tape to the side of the frame that will touch the outside fiberglass wall but not to the part 90 degrees to that the enters the window. I have tried to show this with the arrow in the photo where I am showing the edge of the frame. You can also see where we applied it in the Uhaul
manual diagram where they are applying 'silicone sealant' although they seem to be applying it all over. (Do not use silicone for this project).
I found if you leave the paper backing on the butyl you can press it to the part of the frame you want without it sticking to your fingers, but you need to rip the paper off around the corners to get the butyl to wrap properly. We decided to apply two layers, so pulled off the paper backing on the butyl we had applied and then applied a second layer in the same area. You have to be careful doing this as butyl really sticks to butyl - if you stick it in the wrong spot you need to use a box cutter to cut the layers apart.
Ok - so now you have two layers of butyl applied on the edge of the frame that will press against the outer fiberglass wall once the frame is inserted.
Lift up the frame and insert it into the window - you will be happy if you did a dry run, so you have a sense of how to insert it best. Two people help for this initially. But if it sticks, one person inside can help guide the window.
Once the frame is in, you need to make sure all corners of the window hole are covered by the frame and butyl. Push the window frame to where you need it.
With the frame in the location we liked, we then pressed butyl in to the spaces you can see from the inside between the frame and the fiberglass hole. I had to pull the carpetting around the window by about an inch (and even trimmed it in some spots) so I could do this. Again I have tried to show where we put this butyl with an arrow on a photo of the window inside before removal.
When you are happy with the butyl, you reattach the frame. I used stainless steel Robertson screws of course. Insert them just to hold at first, then go around screw by screw clockwise (or counterclockwise) turning just a little to each. It will take a few round the clock turns to tighten them all up - you don't want them as tight as possible - just to clamp the window. Too tight and you will start to squish the inner frame.
Butyl will start to squish out the outside of the frame - good, this is what you want - you can cut the excess off later with a plastic scraper (to not cut the fiberglass).
I found I was able to tighten the screws a little more the next day after the window had sat in the sun.
Now the window is in. My window has passed the hose test! A great improvement over its leaking all the time before.