[trularin]

rbray, you may have what they call a "single wire setup."

Here is a good test.

Start the car.
remove the positive cable from the battery. Is the car still running? No, bad regulator or alternator. Yes, go on.
Take a voltmeter and read the voltage across the positive cable ( NOT connected to the battery ) and ground. Is it above 15 V, your regulator may be at max and need to be replaced. Less than 13.8V and I wouls say the regulator is set too low and needs to be replaced.

With the engine running, voltmeter connected, turn on the lights. Did the engine RPMs drop a little? Did the voltage reading drop a little? It should. If it was above 15 V and turning on the lights drops it down to 13.8 to 14.2V, Mr.fixit is correct.

Just a thought.