BTDT. The failure in my case was the splines on the nylon gear that's a [poor excuse for a] splined fit on the driver side extension pivot arm stripped so the gear wouldn't drive the extension pivot arm as the gear ran along the track on the inside of the case. It'll make sense when you see it.

JB Weld didn't hold the gear on the extension pivot arm. The W124 and W201 use a metal gear that doesn't fit. The W208 and W210 use a nylon gear that doesn't fit. Thanks to a forum member, I was directed to an eBay listing for a W140 being parted out and got the wiper assembly for ~$50 with no motor. The wrecking yards I asked wanted $250-400 for a used wiper assembly with no motor.

To remove the wiper assembly, remove the plastic box/chamber that sits ahead of the wiper assembly. I think it's held by 5 Philips head screws. You'll have to detach a couple of vacuum hoses near the right hood hinge and some hoses and wires on the other end. Nothing's gonna leak or anything.

Remove the ~10 clips holding the lower windshield trim in place. It's those funny clips that are something like a drywall anchor. Mine were stuck so I sheared off the heads with a blade and bought new ones. By stopping the wipers at near vertical, you should be able to wiggle the center trim piece off the car. The center trim piece might be held by a nut at its lowest point (don't quite remember). Set the wipers back to rest position.

Remove the right arm like you would any old wiper arm. Remove the set screw holding the driver wiper arm to the extending arm. Put some cardboard and towels under the joint. Wiggle the driver wiper arm off the extending shaft. The cardboard and towels are there so the spring force doesn't send the driver wiper arm into the windshield.

Disconnect the wiper motor harness from behind the brake booster. Snake the harness into the wiper assembly cavity.

IIRC there are 3 attachment points for the wiper assembly - by the right wiper pivot, below the base of the driver wiper pivot, and near the motor. With those nuts/bolts released, it's a matter of wiggling the assembly out of the car. What works for me is to roll the assebly about the car's pitch axis rolling the lower forward edge of the assembly upwards and to the back. This gets the right side pivot arm out of its slot, and that's really the only thing holding it in place at this point.

To get into the driver side gear case, mark the relationship of drive arm and the driven shaft, remove the 19(?)mm nut holding the drive arm to the driven shaft, then remove the circlip on the driven shaft. The case should come apart.

If you find the problem to be something other than the fit of the nylon gear on the driven shaft, I probably have spares you can use.

Let me know if one of your front seats won't raise :/

Sixto
95 S420
87 300SDL