Odds are that the brake light switch has failed. Besides turning on the brake lights, it has lots of other functions, and this is a common failure and issue on the W210.

Resetting will work only for a short while.

It's easy enough to change and I'd recommend buying the part at the dealer. There are lots of knock-off/aftermarket switches that are pretty bad and even at a greedy dealer it's only about $25. When you have the switch in hand, drop the lower dash panel (four screws?) and you'll see it up along the brake pedal arm.

If you take it in the dealer may charge you upwards of $300 as they will plug it in to SDS and futz around for a while on it. Of course you get "free" coffee and a "complimentary" car wash.