Auxiliary lighting — work lights, beacon bars, backup floods — is one of the most common fleet upfits and one of the most common sources of electrical fires when it's done wrong. The rules are simple, but they get skipped because the truck "runs fine" on the test drive.
Power every auxiliary circuit through a relay, never straight off a switch. A relay lets you run a short, heavy-gauge power path from the battery to the light and keeps the cabin switch on a low-current trigger wire. Switching 20 amps through a dashboard switch is how cabin fires start.
Size the fuse to the wire, not the load. A 14-gauge wire is rated for 15 amps continuous; fusing it at 20 amps means the wire becomes the fuse. Match the fuse to the wire's continuous rating and the load will always be protected.
Route harnesses along existing chassis looms and secure them every 12 inches with zip ties or cushioned clamps. A loose harness rubbing against a frame rail will chafe through insulation in a few thousand miles and short to ground.
Use sealed, weatherproof connectors — Deutsch DT or Amphenol AT — for any connection outside the cab. Bullet connectors and twist-and-tape joints fail in the first winter. The cost difference is two dollars per connection; the failure difference is years.
Finally, ground each light to the chassis at the closest clean bare-metal point. Daisychaining grounds back to a single battery post creates ground loops and turns the chassis into a current path, which is how you get flicker and EMI on the radio.