Why won't my 93 mazda protege alternator charge battery?

[Marcus M]

i have a volt meter and my battery reads 12.3v and my alternator reads 12.2v and go's down to 12v when my a/c is on and slowly drains my battery.
i bought a new alternator and put it in to the same results so i returned it, i also brought my old alternator to O' reilly auto parts and had them run a test and they said it passed everything and the voltage is where it is supposed to be.
so i think i have it narrowed down to faulty wires, is there any way i can test individual wires?
the guys at the car store said its probably a bad ground, how can i check the ground wires? i took the voltage meter and checked where the battery is connected to the body of the car and that is properly grounded... and the battery is brand new so its not the battery.

 

[Gobyknows]

Your alternator should be putting out close to 14 volts with the engine idling. Trace every wire coming from your alternator and the connections on each end of each wire. They should be snug or tight. The alternator passed O'Reilly's test so you know it is working properly. You have a new battery and your volt meter says its good so it has to be a faulty wire or wiring connection.