The symptoms: your wipers move slow, and in very cold weather may not move at all.
The repair: remove the motor and add modern grease, and a drop or two of oil.
I actually replaced my entire wiper mechanism because someone had put on the wrong size button where the wiper arm attaches. One button was loose when I got the car, and had been crudely drilled for a set screw. The other one was brazed (welded) onto the shaft.
Ideally, start with your wipers parked. Otherwise, you'll have to re-align everything later.
Use a tool like the 45 degree needle nose pliers to slide the wiper arm off the knurled shaft.
Remove the large, outer retaining nut.
The angle is awkward, so I used a smaller wrench to turn the larger wrench.
The nut is loose, and should slide off over the knurled shaft.
This is the motor and mechanism out of the car. I think you can remove the motor without removing the entire wiper mechanism, by removing the 3 10 mm bolts visible here. One bolt is behind the wiper arm.
My motor was fine after being lubricated. Here is the motor after being removed from the mechanism.
1) Unplug the electrical connection, and pull it through the metal between the air inlet and the engine compartment.
2) Remove the 10 mm nut on the drive shaft, and loosen the arm.
3) Remove the 3 10 mm bolts holding the motor to the wiper mechanism. The front two bolts are easy. The rear bolt you'll have to get by alternating the open end wrench from one side to the other. I was only able to get 1/8 turn on each side. Slow, but not all that difficult. Save the bolts and star lock washers.
4) Lift out the motor. Carefully wiggle off the plastic cover.
5) Remove the 4 screws holding the driver gear cover in place. Carefully lift the cover to add a couple of dabs of grease to the worm gear. I added a couple of drops of engine oil too. Replace the cover. Be sure that the gasket seats correctly. There are a couple of plastic alignment pins. Be sure to get the ground wire under the correct screw.
6) Loosen but do not remove the two Phillips head screws holding the motor case.
7) Remove each screw and it's associated anchor, but do not allow the case to wiggle.
8) Pull the case straight off. If you must move it, wiggle in a direction that will force the worm gear against the large drive gear. At least in a VW Beetle motor, it is possible to remove the case at an angle and pull the motor armature out too. This is not a disaster, but is certainly a headache. The BMW motor armature may not come out, but you don't want to find out. There are large stationary magnets inside the case, so it will seem to stick.
9) Lubricate the bearing in the inside end of the case.
10) Reassemble the motor, and plug in the electrical connector for a test.
11) The motor should run smoothly at both speeds, and park. These little motors have quite a bit of torque, and if you hold it while an assistant turns it on and off, you'll feel it twist when it starts or stops. It makes a quiet whirring noise.
12) Make sure that the motor is parked. Put the motor back into the car.
The symptoms: your wipers move slow, and in very cold weather may not move at all.
