Buy it here - probably cheaper than your local stealer.
Generally these are at the back of engine, where the tranny mounts to the engine.
Subscribe to MB EPC online
http://epc.startekinfo.com/epc/ (it is free, believe it or not!)
Look up your car and engine. There's a picture in there when you get to the CPS page of where it is.
If you want to see what it looks like - go to the top of the page and click on buy parts. Pick your model and year. Find crank position sensor (probably under engine electrical). There's usually a picture of the part. All you'll see when looking for it though will be the wires going to it. Most of it is down in the rear engine/transmission cavity.