New alternator and battery, still low voltage.

[nominous]

Stick a wire from the batt (or alternator output wire!) +ve to the exciter terminal on the alternator via a small bulb. Say a 5w indicator one.
Bulb is to limit the current. Should excite it and check with your DVM.
You can't leave it like that tho or it will be on constant.

Then the fault is between the exciter wire and the dash.