If you don't have a Haynes manual and a multimeter, I would advise you to get them. Then use the multimeter to measure the voltage at the connector which fits onto the bulb. There will be a switch possibly a realy, a fuse and there may be several connections between the bulb and the positive terminal on the battery, so turn on the light switch and work your way back along the circuit towards the battery measuring the voltage at each join. When you get a reading of 12v then you know that the cable or component between where you are measuring now and where you took your last measurement is faulty or the connection is faulty. Good luck!
Oh yeah and check the earth connection to the bulb too.