92' lxe weird alternator problem, someone please help me

[Holla McBalla]

I brought my car to a shop that told me they could swap the alternator. I assume it works like a swapping a hard drive or video card on a computer. Unplug/take out not working part, put new part in and reattach wires. This shop put a "used" one in and told me there was something wrong with the electrical system because the battery light was illuminated when the car was running. (I didn't know this at the time but all I had to do was start the car and disconnect the battery to see if the alternator worked) I was in a hurry so I said fine it runs right. The next day when the car wouldn't start, I had a friend hook up a voltage meter to the alternator with the old battery attached and it read 12 volts but dropped to 5 when I tried to start it. I went and bought a new battery, put it in and the car started right up. We then disconnected the wires from the alternator and started the car. Started right up. We then voltage tested the disconnected alternator with the car still running. It read like 1 point something volts. So the garage sold me a bad alternator thinking I would take their word that it was the electrical system. I brought it back and they put another alternator in the car. Same problem except when I was driving home the battery light would come on when I was at any complete stop and sometimes when I was driving. I have not communicated with the garage since that day when I picked my car up. There was nothing wrong with the car before I brought it to them. So I'm looking for some help with what could be wrong. I don't have a lot of money to throw into this car but I need it for school and work which are 10 - 20 min residential/highway trips. I have thought about attaching a new 10 gauge wire from the alternator to the battery to bypass any possible loose connections. Any thoughts on that idea?