Charging / Electrical problem

[72K10]

My battery keeps going dead.
I fixed all possible shorts, replaced both the voltage regulator (high quality NAPA) and battery.
I made sure all ground straps were in place and connected. Even added another between engine and frame.

Replaced alternator last night (good quality NAPA), tested the system; showed only 12V returning to battery at first, but then jumped and stayed at 14.8, right where it should be. Got up this morning, started it up, pulled on the lights, and the ammeter dropped. Lights flickered sharply. Checked voltage at battery, and it was getting only 11.8 volts returned.

Seems to me if it were a short, that shouldn't keep the alternator from sending at least 14 volts back to the battery. Why isn't that happening when I turn on the lights? Why did it return 14.8 last night but less than 12 today? Where would the short be? Seems like light switch would be the obvious place, but again, why would that preclude at least 14 return volts?

Appreciate any ideas / help on this. This problem has been kicking my ass for three weeks now.

Chris