You can access the shut down switch on the ignition switch by removing the lower panel but access is much better and easier by removing the instrument cluster. The cluster is a press fit into the dash. You do not need to remove the steering wheel. I have had good sucess by fahioning two hook wires from a wire coat hanger. Using about 8 inches of straight wire fashion a curved hook on the end of each wire. Insert a wire at each left and right end of the instrument cluster with the hooks towards the front of the car. Push the wires in as far as possible with the hooked ends facing towards the center of the IC. Using a pair of pliers pull on the straight end of the wires alternatively. Be careful not to pull on the plastic face of the IC as it will separate from the IC. The IC should pull out about 3-4 inches. At this point you will be able to reach behind the IC and unscrew the speedometer cable from the speedometer. Once the cable is released the IC will pull out another 3-4 inches and you will be able to remove the wires frim the back of the IC (mark them before removal). Using a 10mm wrench disconnect the oil pressure line from the back of the IC oil,pressure gauge. Pull out the large bundle of wires on the left side of the IC (no need to mark these wires as they only go back one way as a group of wires). Now the IC can be removed completely from the dash. DO NOT start the engine. If you do you will have oil spraying everywhere inside your car. This is the IC removal process recommended by Mercedes (they even make special hooked wires for this purpose but I made a set myself from a coat hanger years ago and they work fine - a friend gave me a set of the MBZ tools but I still use my coat hanger tools (I own 3 W123 cars , a coupe, wagon, and sedan).

The shut down switch is held on by two screws on the top of the ignition switch and has easy access. Hopefully the switch is OK and you have a disconnected or cracked vacuum tube.

Replacement of the IC is the reversal of the removal without using the hooked tools. If you have trouble reattaching the speedo cable to the back of the speedo, under the hood on the left side near where the back of the engine and front of the trans joins is a plastic holder thst holds the speedo cable. Disconnect the plastic holder from its metal bracket and that will provide additional slack in the cable allowing easier hook-up to the back of the speedometer.

Re-attach the plastic cable holder when finished.