2003 6.0 Not charging

Maybe someone here can point me in the right direction. I have a 2003 ford F250 with a 6.0. my alternator is not charging my batteries. i replaced both batteries and the alternator twice. I also change the two wire plug that plugs in the alternator. it charged for a few minutes then stops. one wire from the alternator plug goes to the battery the other goes towards the driver side of the cab not sure where that is going. any idea where i should go from here.

Thanks

check for corrosion at every connection of your battery cables. You could also disconnect the positive cable from the lug on the starter and see if that makes a difference. i had a starter mess up once and cause an excursion not to charge properly.