Hi, like many I have grieved over the potential for my car to either have or develop this problem, yet with one in college and other obligations I needed to wait a while to come up with the real money to make a correct inspection and repair. Once I found a diagram of the way this area is constructed (included here) I realized I could make a reasonable temporary fix which might stop rust long enough to prevent more serious damage, and would stop a leak if the rust were already through.
My car has two small but obvious rust areas about the size of a pencil eraser, one on each lower corner of the rear windscreen, at the edge of the rubber trim. I took a short blade of a pocketknife, eased it down between the rubber and the stainless trim, and cut the root of the rubber seal for a few inches. This allowed me to raise the rubber out of the way, to better examine the rusty spots. They turned out not to have perforated, although it would probably not have lasted much longer. I am cleaning them with a tiny piece of sandpaper, and I will treat with rust-stop chemical and paint with black paint, then carefully stick the rubber strip back in place with black silicone sealant (the Car's finish is black). This should serve until I can do a better job. Fortunately my Car does not leak at this time, and I am able to keep it indoors and therefore pretty dry.
I hope this helps someone feel better about their Car, and gets you by for a while.

 

I had this problem years ago - with perforations in the channel - water in the car.

Tearing the windscreens out is not a very big deal. I repaired the channel with new metal welded in and the entire channel coated with POR 15 - front and rear.

I did not use the "plastic finishers" as I believe that they are a part of the reason for the channels rusting.

Water cascades down the glass and comes to rest behind the finishers and sits there until it evaporates, or rusts through.

I found when I removed the old windscreens, that the factory did not use more than the tiniest bit of urethane sealer - certainly did not fill the channel. That left a lot of space for water to collect. I was amazed that the rear screen did not blow out as there was hardly any sealant holding the glass in.

I used a "lot" of urethane - filled the channel to the brim, then cleaned the excess away using mineral spirits.

Unless you knew about this modification, you would never notice the lack of finishers, in fact if any one has need of these finishers for their car - I have a few sets, still in the plastic bags they came in. Pay the postage they are yours - free.

 

I agree: the finisher, esp the lower one, is holding water; in fact, some water ran out when I cut mine, and the car had been out in the sun all day.
Fortunately I was not rusted through yet; my little repair worked very well, and is not really noticable, once the finisher is glued back in place.
More urethane is as you say the answer. The finisher has a tab or tabs which push into the uncured urethane during glass installation, and this holds it in place once it is cured. Just fill the area with urethane, and there is no place for water.