There is some reset procedure which you can try. I do not have rain sensors so I am just passing the information.

"Resetting Automatic Windshield Wiper Function (with a tip of the hat to mvmiler):

First, pull and reseat your windshield wiper relays (locations are on the fuse diagram at the drivers side fuse / relay box)).

With engine running and all passenger doors closed, starting at #0, turn on wipers to #3 position, then put the switch in the #0 position then put it in the auto #1 (rain sensor position) then have someone splash water on the windshield as the doors have to be closed to reset this function. Then they should function normally.
"

The doors must be closed however, you can try spraying water from the opened sunroof

The wiper relays K1 and K2 are located at the driver side towards the firewall.

Make sure the fuse #4 at the rear fuse panel (under the rear seat is not open.

 

I know this is 8 years old, but hoping the OP will get a notification or someone with an answer. I had my windshield replaced on my 2006 C230 and had the light/rain sensor disconnected. I was happily able to test the light sensor with a flashlight and the interior and exterior lights go on/off. Using the above instructions I was able to get the wipers to automatically come on when I poured water through the sunroof, however once I open the driver's side door the setting seems to no longer work. I retried this 4 times and it always works until I open the door and try to pour water on the windshield and they no longer work. Anyone have an idea to make this setting "sticky"?

Adam

 

It's normal, It is a feature that prevents the driver from taking with the water, so the windshield wipers do not work with the door open