Seat fixed! Thanks!
Thanks everyone. I finally got around to the seat last night. Everyone's comments helped. I couldn't figure out how to get triflow to the latch, so I went ahead and pulled off the door seal to lift the upholstery. I could only lift the upholstery a cm or so--barely enough to see, and not enough to understand. So I pulled the seal from the driver's side (the working seat). Again, prying up the upholstery with a big screwdriver and using a flashlight, and then moving the white latch that holds the seat bottom, I could see how it works. There is a little 1/4 inch diameter metal "button" that goes down about 1/4 inch (or less) when the lower seat is lifted (i.e., when the white latch is moved). A long wire more-or-less connects the two parts. Going back to the stuck seat, I peered in and could see that the little metal button was not moving. So I used a long skinny screwdriver to manually push it down, while simultaneously pulling forward on the seat back. Voila! It took several tries, because every time I let go of the big (prying) screwdriver and flashlight with my right hand so I could grab the seat back, the skinny screwdriver would slip off the metal button. Hint: have a helper move the seat when you are ready.
Once free, I could get at the latch mechanism. It was extremely hard to move, so I generously applied Triflow, especially to the two pivot points that I could now see, and rocked the latch back and forth.
Lesson 1, for anyone with a 124 wagon: lower your seatback periodically and apply lube to the latch that is otherwise inaccessible (I plan to use some lithium grease, too). Or maybe the factory forgot to lube mine.
Lesson 2: warning. I tried Danny's method first, and unfortunately succeeded in dislodging the (RubeGoldberg/MickeyMouse) connecting wire from the lower (white) latch. I tediously reconnected it using needle-nosed pliers, long skinny screwdriver, and a flashlight--and bifocals, and experienced a minor miracle as the wire slipped back into place.
Question: It seems the black tab that holds the seat belt just below the frozen seat latch might also hide a bolt that holds the upholstery to the chassis. If so, and if that could be moved, it would greatly improve access to the latch mechanism. Any ideas?
94 E320 Wagon, 170k, totalled in 2006
2002 BMW 525i Wagon, 75k
1991 BMW 318is (two of 'em)