Size the cable to the current and the length, not to what fits the connector. Voltage drop over a long run dims the lamp and heats the cable, and the symptom presented to the dealer is a dim lamp, which sends the wrong part back under warranty.

Fuse to the cable, not to the lamp. The fuse exists to protect the wire from becoming an ignition source when it chafes through, so it is sized by what the cable can carry, not by what the lamp draws.

Use a relay for anything beyond a small load. Running a light bar through a dash switch puts the full current through a switch that was never rated for it, and the switch fails hot rather than open.

Protect the entries. Grommets where the loom passes through panels, strain relief at the lamp and a connector that is not left face-up to collect water account for more surviving installations than any other single decision.

Supplying the harness with the lamps removes all four decisions from the day of installation, which is when they are most often made badly.