Never use a wire nut on your truck... (yes, some people actually do!) They are not designed to be used on SAE stranded wire and can cause short circuits and fires.
Instead, what you should be using is a high quality mil-spec crimp/shrink connector with adhesive lining. We are experts when it comes to automotive electrical systems - our repairs last. For pennies more than the cheap vinyl connectors you find at places like Radio Shack you can get a connector that won't fail. Do it right the first time with our connectors and save yourself from a frustrating break down. Available in mixed assortment and bulk packs.