You can use one of the wires from either brake light and make yourself a good ground in trunk and running the two wires to you light switch. be careful where and how you run wires around the trunk hindges as you do not want them to become pinched and short out as that is a safety issue and get you pulled over. always solder and heat shrink all splices and wire to wire connections, if you dont you will hate your self later as wire from front of car to back of car could then need to be replaced.

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