Don't spend $1000 on a new motor till you try this cleaning and lubing procedure! I had the same problem on my 94 C280 a couple of weeks ago. I found this DIY tip at a list faq's I subscribe to, and so far and about 3000 miles later, so good.
before replacing the wiper motor, try spraying the shaft where the arm slides on. Turn off the ignition, turn on the ignition key back on to use the wiper till it reaches a quart or half-way, stop the wiper. Then take the wiper arm and spray it with a lithium lubricant, i had the same problem.
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