Alternator not charging

Looking for help! My 74 Dart sat all winter indoors and when started for first time this spring
alt charged for short bit then stopped. I have done the following.
New Alternator since old one was making noise anyway
New regulator
New ballast..4pin
New ground battery cable
Refreshed all grounds to body and engine.
I have the wiring diagram and have found no bad wires yet! I have not yet checked wires as they go through firewall. Anyone know which wires through firewall allow alt to charge?
Main power feed from amp gauge has good connection through firewall.
Do you have a free Harbor Freight voltmeter?