1997 Nissan XE pickup problem. Please help!

Truck is a 1997 Nissan XE pickup automatic 2 wheel drive 4 cylinder. Went to start the truck one day and it wouldn't start. We replaced the battery and it started up and ran for a couple days, but then wouldn't start again. We had the alternator checked and it was bad, so we replaced it with a brand new one. Ran fine for 30 days then crapped again. Took it and got the alternator checked and it was bad again, so once again we replaced it with a new one. 20 days later it won't start again! The belt is tight...This truck is eating alternators! What could be wrong with this thing?! :banghead:

Update** we just checked the battery and it's reading 12 volts! But when you try to start it it just clicks.

Hello, I know this is old, but in case anyone needs help, the two prong connector on top alternator is bad design. When driving , it don't stay tight. But at idle parked they seem okay. Mark which wire goes to each terminal, then cut off connector and install two single connections with tight spade terminals. Solder, or heat shrink, and insulate with rubber covers. Also, check your batt warning light on dash. It should come on then go out at startup if charging. If the bulb burns out, you won't get a warning light, plus a burned out bulb prevents the whole charging system to not charge. There is also a battery warning lamp relay on driver side fender near washer resevoir. Also, check fusible links at battery terminals. They are awful. Lastly, its a great job idea to upgrade battery cables and have a direct cable from starter to battery. And upgrade the alternator to battery cable and put in line fuses instead of fusible links. So you can easily check in the future. Good luck