Maybe this will help someone. I'll keep this as short and to the point as possible.
I have a 2010 (pre facelift) C300 with the faulty headlight switch that has a mind of its own. I recently also changed the fog bulbs to LED which implies I want to always use my fog lights at night (I generally never did in the past but here in the northeast, local government has turned off half the street/highway lights).
-Didn't like moving the position out of AUTO, in general, but with the headlights turning off and on intermittently at night when I used the fog lights, etc., I gave up. It truly can be dangerous. Not just since you lose your visibility but driving on local streets with intermittent headlights may indicate to other drivers that you're letting them in, etc.
-Bought a used, facelift switch from ebay. Didn't realize it's not compatible due to the plug interface. And don't want to pay around $200 for a new, pre facelift switch.

My solution:
-Installed a fuse tap and ran wire and tapped into the fog lights. Found a fuse with 'key on' to tap into. Now I can leave my switch in the AUTO position and I guess I now have homemade DRLs. (I don't use the DRL factory setting on the dash - I always thought DRL meant, or should mean, just headlights/accent lights like my old 2001 VW Jetta. But apparently it also turns on tail lamps, side lamps, etc).
-*Originally, I tapped into the side marker lights (and the LED fog bulbs I bought didn't give me a bulb-out dash warning), but when I tapped into the side marker lights, they did. Maybe it's because the side market bulbs are also LED but not sure, didn't want to bother with resistors and spend too much time on correcting the issue - I'm not an electrical engineer.

The other thing I almost tried was to take the switch apart and clean it, which I believe some have. It's not guaranteed and was afraid to take the switch apart in case I couldn't put it back together.

Maybe this is of use to someone out there.