With the engine running all electrical loads should be running off the alternator output directly. In the muscle era mopars tapping off the battery put's that load thru the ammeter and the associated wiring everyone seems to think is bad. the only load that should be going thru the ammeter with the engine running is the battery charge. Anything else renders the ammeter's reading useless.

Tap it off the alternator stud, that's what I do when I run relays.