That's the ASR switch. It's not an ASR defeat switch but I believe you are supposed to punch that button when you have snow chains on the car. Someone will have an owner's manual handy. It's my understanding that all 124 500s have ASR but it was optional on the 400s/E420s, which is the reason why I have the latter. I will not own a car with ASR - nothing but trouble, and usually inexplicable trouble.