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The black wire would be negative/ground. The other wire would be a connected to a switch. Trace the originals back to the source. The black wire can pick up the negative anywhere along the route but I find it best to run them parallel from the source. There should be a negative buss bar or stud close to the switch. Use heat shrink marine butt connectors and I find melting the shrink easily done with a wind resistant lighter/log starter.
Garris