Rear quarter panel glass trim...is there some state secret to getting the trim into the rubber seals...

 

This video from @Dean_Fuller covers it really well:

 

Unfortunately the windows are installed...

 

At the end of the day, as nuts as it sounds, you'd spend less time removing the rear seats, rear console, rear quarter panels (let's say 4-6 hours of work) then R&R the quarter windows and install the window trims, than to struggle to get them into place in situ. Caveat, I haven't done this on an S or an S3 with the rear spoiler, not sure how much extra time that adds.

There are folks on this board who say they have done it, and I believe them, but I was never able to do it myself.

 

This is from memory - YMMV and no warranty! Applies to an S4 and probably to all. You will need 12" or so of string trimmer filament or similar to temporarily spread the channel open while you seat the trim. The object is to stuff a short piece of the string into the channel and pull it along the full length, pushing the trim into the channel right behind the filament as it moves.

1. Wet the channel in the seal with soapy water to lube it
2. Embed about 2" of your 12" filament in one end of the channel in the rubber seal to spread it open
3. Position your trim piece ready to push into the channel
4..Pull the end of the filament ~1/2" through the channel
5. Where the filament as vacated the channel, press the end of the trim into the channel to get it started. It is obvious when it seats properly.
6. Drag the embedded portion of the filament along the channel by pulling slowly on the filament free end. (This requires some steady strong force)
7. Following the path of the filament as it spreads the channel, push the trim into the channel 1/2" at a time

Go slowly. Two people will make it easier (one to pull the filament, one to seat the trim in the channel). Keep the channel as slippery as possible. To save time, try to keep the filament from fully popping out of the channel as you drag it through. But if it pops out, keep your finger on the trim that's done so you don't have to start over, stuff the filament back in, and keep going. You will quickly get the hang of this.

@Rob Edwards I wasn't sure if it was OK to laugh at your analysis but if not, sorry! We replaced the seal with 2 people and no removals of any interior parts. The trim seating took about an hour to figure out and maybe another hour execute on both pieces. The whole job was more than a full day, mostly scraping off old terrible PO caulk under the old seal and painting. Why do those windows s**k so bad???

 

Totally ok to laugh at my analysis- what you describe sounds like it should work and it's wayyyy easier that disassembling the back half of the car. I have done 5 or 6 cars' worth of quarter windows and it was always in the context of a windows-out restoration so I clearly have some blinders on here. I did try, once, to do what you describe and wasn't successful.

I have three teenagers- laugh away, I can take it.