This should be posted in "Tech Help", but since I also own a 300E, I will tell you that it should be written in the radio operation manual provided with the owners manual.

Keep it handy, as you have to reset this code every time you disconnect the battery for an extended period of time...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car