1992 Chevrolet Cavalier 3.1L charging system

I recently installed a new alternator, battery, starter and serpentine belt in to my 3.1L. But the charging light is still on and the alternator test that it is not charging while in the vehicle but when you bench test it all the new components are working fine. The car starts and runs but as soon as the battery gets low then it starts to make the gauges bounce and eventually the car dies. I assume that it is something in the wiring harness but have not been succesful in loacting the problem. The diode also test good.

Does anyone have a suggestion because I have never been stumped like this before on a repair?