Second 12v (C. Lighter) wiring

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Some simple things can help you go along way:

1) Listen to TTT
2) Use a multimeter to know the voltages and resistance you're dealing with--remember: V=IR
3) The more resistance on the wire, the more heat. The more heat, the more likely fusing will occur, the more likely fusing occurs the more likely you'll short out near by wires.
4) Following voltage back to a relay usually is the best bet or wiring in your own relay directly off the battery would be ideal.

Wire in a simple accessory switched wire to the relay (this will close the circuit) and you'll draw 12v directly from the battery and you can specify what wire gauge to use. This also will allow you to put the proper fuse inline. When you turn the accessory off (the circuit is then opened), and your relay will keep your electronic device from drawing power directly from the battery.