1999 Mercedes-Benz S500

Hello, This is a fairly common problem for Mercedes. You can try to lubricate the switch with a graphite powder for locks that you can find in almost any auto parts store. Spray the powder into the lock cylinder and work the key in and out of the switch, then try to turn. Do Not Use Any Sprays Like WD-40 etc. If this does not work, order a new key from Mercedes, and try again. If you do get it turn, make sure that you can get the car to someone to remove the lock cylinder because it will do it again! If you can't get it to turn, you will need to take it to someone that knows these cars for repair. It is an expensive repair and you don't want just anybody to do the job.
I hope this helped.
CYA, Gator241