When you turn your wipers on inside, can you hear the motor running, with no movement?

If so, you need to look under the cowl just in front of the windshield.
There is a ball and socket coupling between the wiper motor and the wiper "transmission" linkages that likes to rust and eventually the socket falls off of the ball.

If you find that to be the problem, you can semi-permanently fix it by drilling a hole in the top of the ball (through the socket) and installing a screw with a washer under it.
A thick coating of a water displacing grease in the socket helps to prevent further wear.

The proper fix would be to replace the whole wiper transmission, but it was ~$400 if I remember correctly.

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...