When I bought the car, the dealer said that I needed to go to a Mercedes dealership to have a new key made because it was "too thin". He seemed to know just how to get get it to turn, but it takes me a long time so I've been leaving it in the ignition. So long as I don't take it out, it turns and starts everytime.

I did a little research and found that this is somewhat common, and that it may have to do with the steering wheel lock despite the dealer assuring me I just need a new key.