I know absolutely zip about a 97 E420, but I'd say you have 2 choices here: get at the back of the lighter socket and re-connect it to an unswitched power source. I'd be mindful of the fuse in the chosen circuit and the load from anything else on the same circuit. The other thing you might consider is picking up an inexpensive female socket (Walmart has them for about $5.00), and wiring it directly to the battery. Perhaps it can be located in an inconspicuous place (under the front seat, eg.) so you can just pull it out when you need it. I'd assume that would be when the car goes into long term storage?

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1986 560SL
2002 Toyota Camry
1993 Lexus