If the key is stuck in the "on" position, the only other quick alternative is to disconnect one of the battery terminals. Inasmuch as it is now Tuesday 12/16 and your original note is from Sunday, I hope that you found some way to get the key out. On the subject of keys, etc., recently while having a local locksmith carve me a new key for my E300, I was cautioned NOT TO USE any lubricants, liquid or powder in the keyways.
For the boards info; the key was stuck and would not turn either way. To stop the car I first disconected the battery (good idea but once the car is running and the alternator working the car will run without the battery!!!)
next try and pull the coil wire, next try and locate fuel pump relay, finally took the top off of the air cleaner and stuff a rag in the intake! What makes this really kind of funny (sick since of humor) it was my girlfriends car and I was not there, We did this over the phone, while I was trying to locate things on the internet, I was talking to her on the phone trying to explain this (she is an excellant sport and understands mechanical things)
She tried to unplug the coil wire and got zapped! (learned some new words!)
she was using rubber coated pliers and still got zapped! The fuel relay is buried behind the battery, finally I said to remove the top of the air cleaner. The car is back and running, ended up to be a worn key!
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