99 cougar won't start anymore. HELP!?

I have a 99 cougar and about a year ago my brother was fixing my brake lights. He was getting ready to fool with the wiring near the lights (or he might have.im not sure) but ever since he fooled with the brake lights it hasn't started. Just a few weeks ago we bought a new alternator and a new battery. Its wanting to turn over now. It makes a loud clicking/grinding noise but it does not want to fire up. Also the lights and everything comes on now to. And the starter is good. I had it checked. I have a v6 and its automatic. I hope someone can help me out with fixing this. I need it for college. If there is anything else you need to know just ask. Thanks. (: