Either run a fused wire from the battery, or tap into a constant 12V wire somewhere inside and connect to the cig lighter/radio. The ignition wiring comes to mind for a constant 12V source.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD