Factory style Butt Splice and 1 wire Alt question. Proform 140amp

[CopperC10]

I have read many many threads including Mads write up on this topic. I have used the search button alot and I need your help. Maybe Vettevet can chime in here.

We are cleaning up the wiring on the 68 and we are at the factory style Butt Splice. I'm going to call the previous owner "dude" to make it easy.
All the gauges worked in the truck when we purchased from dude. I do not know how considering what I found when i untangled the rats nest.

I wish to use that Proform 140amp 1 wire Alt and retain the gauge inside the truck.
I just run a 8 gauge wire from Alt to the butt splice where I also tied in the external regulator red wire. The brown wire went to harness not to Butt Splice. Here is where i am confused. The Blue and White wire were snipped so how was the gauge in the cab working when we purchased from dude?