Should be the M103 engine. It's located right between the block and the fan. The distributor cap should have a plastic cover over it. It's held on by two tabs, pull out on either side of the plastic cover and pull up. This should expose the plug wires connecting to the distributor cap. You should probably replace the distributor cap while you're at it. Unplug all of the spark plug wires (marking them if they aren't already). I think the distributor cap is held on by three 5mm hex bolts. You'll need either a 5mm allen key or hex bit. The bottom screw is a little more difficult to get at. I used a 5mm socket hex bit with a universal joint adapter. After removing those three screws, the rotor will be exposed. It's held on be three screws. I think these are also 5mm. Remove the screws and install the new rotor.
Here's a shot of my old distributor cap.
OK, so I've forgotten how to add pictures and the interface is not intuitive.
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino