Wiper only goes fast and sticks sometimes

I just experienced the exact same symptoms (only working on fast setting and not returning to park) on a 93 300CE and it was a problem with the wiper motor itself. I was lucky in that a few love taps with a hammer had it working normally again. Likely just a little corrosion build-up on the internal contacts. While not recommended as a permanent fix being it is a safety issue it may help to get you by since at least in my case none of the local stores here stocked these. Servicing the motor is not too difficult. You should only have to remove the cowling and job time is about a 1/2 hour. Part cost will likely be in the $100 range. Hope this helps.