Carry a lighter. The door locks freeze on both my 116 and 123 when it gets really cold here. So I keep a lighter in the garage and in my desk at work. Just let the flame lick the underside of the door handle key housing and it will warm up the lock just enough to turn it. Only problem is if you live in an area where the wind blows it might be tougher. Been doing it for years.

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'80 300SD - '83 240D - '00 E55 AMG - '02 G500