I bought a used 2002 Frontier in March of this.

I bought a used 2002 frontier in March of this year. Tried to start it one morning and the battery was dead. Not knowing how old the battery was I replaced the battery. Shortly there after I started noticing a squeeling sound from under the hood. Then on the way to work one morning I started loosing power, lights were going dim, barely made it back home. I am thinking alternator was going bad when I had to replaced the battery so I replaced the alternator. Since then I have gone through two new alternators both made in Mexico. Is there something that can make an alternator go bad? Am I missing something here? What else should I be looking at? Thanks for your help. David O.