I am going to make an assumption that you have a DVM or Multimeter and know how to properly use it...
Use a voltmeter to check and see if you have 12 volts at the light.
Good 12 volts, disconnect the light from the wiring. Remove the bulb and check the resistance, it should be less than 500 ohms. Check the wiring - the black wire should be ground, check the resistance between the back wire and a bright, shiny metal spot on the body. You should see less than 1.5 Ohms.
Here is what it looks like. I only have one wire. Where does this wire screw into? Also, you will see there are holes for 3 bolts and I only have 2. There is a 3rd screw hole in the casing but not in the roof. Is that normal?