Where is the power supply wire that goes into and out of my alternator?

I have a 93 chevy lumina apv 3.8L. The lights and everything kept dimming down so we tested the alternator and it was weak so we replaced it, when we did that we found out that there are 2 different alternators for my van-a 105amp & a 140amp. There was the 105 on the van so we replaced it with the 140 due to my van having the dual heat it said we should have the 140 so we put it in and it started up and was running great except the alterantor was making a funny noise so we exchanged it then the van wouldnt start. We jumped it and it started but only ran for about 20 mins. So we replaced the alternator again except with a new one instead of a remaned one like the first 2 were. So the van still is not running. We charge the battery 100% and it will start fine but will not stay running and then the battery is dead again. We have power going into the alternator but it's not recharging the battery. Any ideas? We were told it has to be a fuse or a broken connection. Ive know been with out my vehicle for 2 weeks.