Older Mercedes from the 60's.70's had what they called a gorilla knob on the dash.
1. Push it in stops the engine
2. Neutral position engine runs
3. If you pull it out one position its the glow plugs for as long as you hold it
4. Then if you pull it one more position it operated the starter
The only thing the key dose on these cars is lock/unlock the steering and turn the electrical system on. You can remove the key lock up the car and leave it running as long as you want. I had one with an air leak in the injection system that was a bear to start even warm so I would take it to the store and leave it running with the doors locked no key and no steering!
1987 300SDL 6spd manual om606.962 swap project
1985 300td euro 5spd wagon running