I just did the job on my '90 and thought I would share for anybody who mat have some questions. The donor was a '96, so unfortunately it has black trim and handles so it doesnt satisfy my craving for chrome, but it functions just as intended so I forgive them for not being shiney!
When you grab the donor windows make sure you take EVERYTHING: window, hinge channel, seal and backing plate for the opening swivel. I even re-used the same hardware since it was in good shape from a newer van. If you want to tint the windows, this is also a good time before you install them, but they remove easily so it isnt that big a deal if you want to wait.
First before you remove the fixed window, run a plastic tool, I used a putty spreader, between all the seal's surfaces. that means between the seal and glass and the seal and door on both sides, this will free them from the bonding that may occur over time. Remember, you probably dont want a thousand glass cubes on the ground!
from the inside of the door, use the spreader to pull down at the seal close to a corner and I just used my fingers to pull down the corner and some more of the sides. From here you can push lightly on the glass and keep pulling the seal away from the channel it's fixed onto.
Keep doing this and the glass will come out quite willingly and you will be left with this:
Notice that some holes are alreadt drilled for you, but not all of them. We will get to the drilling later.
To access one of the hinge channel holes on top of the door you need to remove a plastic cap, note the direction it goes for re-installation.
Sorry for the fuzzy pic, my camera likes my car more than my van! The 2nd hole is not covered, you just need to push back slightly on the seal to get your deep socket into it.
Install the Hinge channel with the bolts going up through the door on the outer holes. When both are installed and tight, drill the two centre holes for the screws.
Now you're ready to install the seal top first so when you get to the bottom you can mate together the end peices using the rubber bit thats in the hollow part of the seal.
You can either press the seal in firmly or lightly use a soft faced mallot to mate the seal firmly to the channel. This is where you should now be:
Once again, the camera likes the car better than what I want to take a picture of, but you can see how the window fits into the hinge channel.
Tile the window uo high and fit the channels together. You may have to press up as you swing the window down to it's resting position, it might take a few trys but it will end up swinging down if it's aligned properly.
So you're saying "I've gone this far, now what?". Well this is what, you have to attatch the mechanism to the door! The metal plate goes behind the door and is secured by 3 screws.
Congratulations! You now have windows that open and have serviceable spares for the rear doors if you want to keep them solid! This is also a good time to tint the spare windows, so you can install them already tinted into the rear.
Enjoy the feeling of fresh air!