Butt connectors
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On anything outside I use shrink tube and the bare butt connectors. I crimp it and then heat the butt connector and fill it with solder before putting the shrink tube over it. I've found just crimping will sooner or later result in corrosion in the connection. Moreso in the winter especially if you're in an area that salts the roads. You wouldn't believe the number of corroded crimped (and sealed) connections I fix every winter on tractors and trailers. If you're not going to solder it at least put some dielectric grease on the wire ends before putting them in the butt connector and use one that has shrink tube on it or extra shrink tube.