I hard-wired a 700W inverter directly to the battery. I fed the wiring through the firewall, and included an in-line fuse. Consider a set up like that if you don't want wires running all over your ashtray / center console. It all tucks neatly in the passenger footwell or underneath the passenger seat. No issues after 2 years...

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-1985 300TD - 324k mi on chassis. MB "Tauschaggregat" 617 motor + 4-speed conversion
Alaska Roadtrip 2009
-2002 E320 Wagon - 197k mi
-1998 E320 Wagon - 310k mi - retired