After 17 years, the rules of thumb for diagnostics are still the same:
Always start with the basics, testing the bulbs or fresh bulbs, cleaning/inspecting/testing bulbs sockets, wire plugs, and wire harnesses for damage or corrosion. Locate all the grounds, clean them and snug them down. Inspect the wire plugs and optional relays located behind the spare tire and plastic trim, leaky windows and rusty fenders allow water to contact non-sealed wire plugs. Always suspect any trailer light wiring. If any symptoms seem to be related to a short circuit in the power windows, radio or tailgate, check the driver door and tailgate wire harness for faulty or damaged wires.