A couple things to check...
First, check voltage at the battery with the car off, then at idle and then at a high idle(~1000 RPM) See what you have and if it changes at all. Somewhere between 13.5-14 volts at high idle is what you are looking for.
If there's no increase in voltage with the engine running, now you need to start checking a few things.
Test the battery. If you don't have a load tester, you can either take the battery to your local Auto Zone type shop and have them check it.... Or a quick and dirty way is to charge the battery and check the voltage with the car not running. Turn on the headlights and see if the voltage drops off fast. A bad battery won't charge.
If that's OK....Check battery cables to be clean and tight. Both ends of the cables, not just the battery end.
As said, check the voltage regulator for good ground and that all the wiring is tight, clean and installed correctly.
If all that looks good, unplug the field connection from the alternator. Make a jumper wire that runs from the field connection to the battery connection on the alternator. Check the battery voltage first, then start car and immediately turn on headlights. Check the voltage at idle and high idle. If there is no increase when it's running, that points to a problem with the alternator. If the voltage goes up, it points to a problem with the regulator. Don't run the engine for long like this as it may be overcharging the battery. This is a test procedure almost straight from the FSM, but you are using the headlights as the load.
If you don't have a FSM, download one here:
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