What an idiot

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I use both butt connectors ( marine grade ) and solder - depends on where it is. Both will get heat shrink over them.

With solder - using flux is the key - plus a hot solder tip.

Once wires are hot and flux has smoked off - solder should run easily. The flux cleans the wires so solder " takes ".

Other key is right size solder - I like very thin solder for wires 14 to 18 gauge ( most common on boats ).