If you want "all the time" power first make sure the cig lighter is on "all the time". I needed that in my Toyota and found out that it was only on with the key on. I never checked my Mercedes since I don't smoke. In the Toyota I ended up tapping into another source, in that case a wire that powered the puddle/marker light in the door. The radio should have a constant source as well as certain lights. It will be helpful if you have a wiring diagram. Maybe someone else has a more specific answer.

Mike

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra