If you have a battery charger and some wire, set the charger up, wires in the clamps. Negative, strip it back long enough to wrap around the base, or, clip the negative charger clamp straight on it, touch the wire from the positive to one of the buttons on bottom of the light, then the other.
Or, use an ohm meter. Learning this stuff is intimidating, but once you get it, you're gonna be shocked that you didn't understand. You hang in there, don't get impatient or discouraged.
Hold the bulb up and look. If you see two filaments, then one is brake, the other is turn.
Look at the stock bulb, see how it's wired, which filament gets lit up when you hit the brakes?
If the mounting bolts are the ground, make sure that they are grounded, not insulated by paint. You can even run a No Kidding ground wire from chassis to the mounting bolts.
http://www.elecdirect.com/catalog/crimp-wire-terminalsOr, solder a wire to a washer.. Buncha ways to skin this cat.