Here's what I tried yesterday.

Engine not running, but key to #2 position.

Switch wipers on to intermittent - no movement.
Thinking there might be a dead segment in the motor, tried "helping" the wiper just a bit (less than 1/2" movement before resistance felt and I stopped) - no change.
Switch wipers to low speed - no movement.
Switch wipers to high speed - no movement.
Switch wipers back to intermittent.

Check for voltage at all 4 wires going into the main relay. 0 volts at red, yellow and green wires (brown is ground, from what I remember in the schematic).

Double/triple check everything. Pull fuse (#4) - all is OK. Replace same and check for voltage across fuse - all is OK.

Disconnect and reconnect wiper wire harness from main relay - no change.

Reach through driver's window to turn key off - first switch off wipers, but accidentally rotate switch wrong way - to low speed from intermittent, instead of to off.

Wipers start working....

Check all speeds - everybody's happy. On and off and on and off. Everything still copasetic.

Pull out the meter again. Check voltage at red wire. On low - around 7 or 8 (or was it 8 or 9) volts. On high - 12+ volts.

I think this points to the main relay. I'm going to call MB and see if they've got one in stock. I'll get it and keep it until the wipers fail again, since it's a 30-second swap job.

In the meanwhile, a good dose of RainX on the windscreen will at least keep me from wrecking the car should the wipers fail in a heavy downpour.

Please let me know if you think my logic is sound.

Regards