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Even though my HID headlight takes 1/3 the power of a Halogen, it wouldn't work with the stock 40 year old setup. They sent me a relay kit that is a dedicated circuit. I have relays for my horn and coils too. The wire itself will build up resistance over time. I've grounded my loom in forward areas and ran a new main lead. Next is a new ignition switch.
Steve

I don't like to solder too many connections. The heat weakens the copper and the flux can sometimes lead to corrosion. I've found, over the course of decades, a good, solid crimp lasts longer than soldered lug connections, especially if there will be mechanical stress or vibration. I will however solder wire to wire to make a T joint, but that will be wrapped within the harness so all of the other wires will be supporting it, and there will be no stress from disconnecting/re-connecting a device etc.

I've also never had an issue with lighting or ignition needing a relay as long as the wiring was in good condition. If the wiring is in bad condition, I like to fix/replace that.