It's a window lift channel, yes it had rotted (and been removed), and no, you can't get them any more (MTC8478)
However I bought some front window lift channels, drilled out the spot welds, and bodged up a replica of the rear. Bottom item on my bench here is the original rear from the other side (which was pretty sad but at least gave me a pattern), a front, and my "new" rear made from a front. A bike inner tube worked in place of the missing rubber.
Please excuse my welding. I'm an IT sales guy, not a welder, this is only the second time I've used my welder, and I did'nt really care about the finish as it's hidden. I refilled the drilled-out spot welds with weld and it looks OK. Obviously I made two and replaced both sides!