My battery light on my dashboard will not go off on 2000 Mazda Protege and I need some possible answers.?

I took my car into a shop a few weeks back and after they ran their diagnostics they determined that it was a faulty alternator. I went and purchased a new alternator and after I installed it the battery light turned off. Today the battery light reappeared. New alternator and a new battery are in the car. What could be causing the light to reappear and possibly drain power. Thanks