I did the fix this afternoon - happily, as Klaus suggested, it was the lower channel in which the window runs during retraction (thanks for the heads-up, Klaus). It was a relatively straight-forward operation - again, the pictorial from Bay 13 was a great help, with only a few pointers to add if someone else should have to do this thing.
First, the break in this case happened at the small brace near the bottom of the channel, which is held in place by the #25 Torx screw in the door's rear edge. If this turns out to be your problem, I'd suggest removing the channel before tackling the screw removal. The replacement channel has a beefed-up brace (methinks that there have been lots of these problems) but unlike the old one, there's no integral nut to run the screw into. Therefore, hang on to the piece of the brace still at the end of the screw as it is removed, and this will serve to take the screw when securing the new brace.
During the installation, it's handy to have a mirror-on-a-stick and a long, fine, straight-tipped screwdriver, to guide the end of the rubber lining of the upper channel into the top of the lower - this'll become maddeningly clear when you have the replacement part in your hand.
Finally, I used two strips of masking tape with corresponding arrows - one on the window lower corner (rear), the other on the upper corner of the door liner: there's a lug on the liner which fits into a channel on the door frame, and you'll find that the whole linnr installation goes smoothly if this device is properly lined up FIRST. The door lock pull will sway back and forth a fair bit, so can't be relied on for accurate alignment.
Oh yes - have lots of the plastic clips on hand - the liner had to come off one more time to re-install the plastic cover protecting the lock area.
I hope that this helps anyone who might experience the root problem - considering the change made to the channel brace, I'd be inclined to expect it eventually ( my wagon just turned over 188K Km.). I guess that's about average.
Cheers,
Garth.