Is the rear wiper motor definitely at fault? A common problem with W124 wagons is that constant flexing of the wiring harness into the tailgate causes the wires to fatigue and break. Generally, the heavy gauge ones fail first (ie. chassis/earth and rear window demister). I have had to repair the harness in our '90 300TE. By pulling on the individual wires from the end where they terminate above the roof lining above the load area you can identify the broken wires when they pull out. With the roof end of the harness disconnected, it is possible to splice new wires into the harness, although it is an awkward job. I replaced the two heavy gauge wires with paralleled lighter gauge wires in the hope they might withstand the flexing better.

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107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.