How do i reset the mercedes rain sensor?

The first thing You're going to want to do is pull out and reset the windshield wiper relays, which are located in the Mercedes' fuse panel, on the driver's side, in the engine. You'll find the diagram of these usually located in the tool compartment of Your spare tire well. Typically, the wiper relays are going to be K1 and K2, and they're close to the driver side firewall. After You've done that, and with the engine running and all the doors closed, start at #0 and turn the wipers to the #3 position, then put the switch back to the #0 position, then put it in the automatic #1 position (the so-called "rain sensor position"). It's best to do this while it's raining... Otherwise, You'll want to have someone toss some water on the windshield. In a pinch, You can open the sunroof and spray liquid that way: If You don't have water handy, the content of said liquid is left as an exercise for the driver.