most of the lights that I've worked on I start with these steps:
1)check Bulbs.
2)check Fuses.
3)check Grounds, they get dirty, rusty, and break the conection.
4)check power.
From there you are into a good old fasion electrical trouble shoot. Checking sockets, wires, rub points on the harness, ect. Start from the battery side of the problem, and chase the power back to the problem untill you don't have power any more, pin point the problem and replace accordingly.
Good luck.