There is a black/w white stripe wire that I assume is for the ALT light in the gauge cluster plug. My new harness does not have this, and also I am using an internal regulated alt.

How do I wire up this light to show when alt is not charging?

Need a bit more info:
1.] You are using a 3-gauge cluster with a GENERATOR light?
Because the 7-gauge cluster does not have a light, only an amp gauge.
You will have to add a light for this type of panel.
2.] Is your alternator a 1-wire or 3-wire version, because to wire a light in
requires a 3-wire alternator. There is a way to wire in the factory amp gauge though with either version.

Are you wiring a factory harness or painless?

Found out what it goes to, it actually connects to the big red wire in the engine compartment that is a three-way split (one to the alt, one to the regulator, one to the +junction box next to battery...)

I am using an internal regulated alt, 3 wire version, and I am using E-Z wiring 21 circuit harness...