You'll have to remove the gasket in order to do this with ease.
On the older models there MIGHT be a screw just on the side of the frame where the door opens. This will release the plate that overlaps the gaskets end piece.
If there is no plate then just continue to pry away at the gasket making sure how you took it off because it could be tricky sometimes to replace.
Once you've released the end, roll down the rear window and standing from the outside push against the panel to release it from the clips which are attached to the frame.
You can do one of the following: Either pull up on the gasket and bend it behind the rear head rest so you can momentarily install your piece and then replace the gasket in place.
Or you can remove it completely by starting from the end you originally pried out. One smooth motion give it a nice tug and slide it out. To replace put some of your bottled solution under the gasket that slides against the frame so you can slide it back in place with ease.
After that just push the panel back into place insuring it is completely snapped in place by the metal clips. Then start working the end of the gasket back into the crevices of the sidewall.
Next time I get one of these e350 coupes with this window I will do a write up on it.
