Maybe different on my car--a w124 E320 1995--but I found the rear panel is exceptionally easy to remove and everything is relatively easy to get at--Another idea of why your wiper has a problem is this--does yours have the automatically engaging feature when you back up and the front wiper is on it automatically turns on the rear? If this circuit has failed it may not be possible to turn on from the front.

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2002 Mercedes-Benz E320 4Matic
1995 Mercedes-Benz E320 Wagon
2005 Land Rover LR3 V8 SE
1999 Audi A8 4.2L quattro