Hi, wire nuts are for houses, [my opinion] because low voltage equals high amperage and loose connections melt. Twelve volt systems need a good solid connection. I use crimp connectors, a Thomas & Betts professional crimper, shrink tubing, and a propane Barbeque lighter to shrink the tubing. Also black plastic electrical tape is known for coming loose in hot weather. I have a roll of special Ford factory electrical tape, [not plastic] but I only use that for making looms. Click on my blog to see how my brake wires were put together from the factory, except one.
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