Chrysler Repair: Chrysler issues, chrysler 300m, 1999 chrysler 300m

Question
We have a 1999 Chrysler 300M.  The battery light came on and we thought the alternator was going out, so we changed it with a "used" one. Then we figured the battery was bad because the car kept dying, but would jump and drive. So we brought the car to autozone to have them check the power output with the car on, and it dies quickly like the alternator or battery is still bad. BUT we took both out separately to get them checked on the machine at the store and they both "passed". We now think there is a faulty connection somewhere and we don't understand where to begin. Any help would be appreciated...

Answer
have u checked for blow fuses by chance? There is also a fuse link between the battery and the alternator. it should be green. If u pull the link and it is bad it will be spongy feeling  a good link will be hard to pull like a normal wire.
I live in the Atlanta area I would be glad to look at it for u.