I have a 1988 dodge 3/4T Pickup Voltage was above 18.1 at Idle, Auto Zone said that the internal Voltage regulator was bad in the Alternator. I replaced the Alternator, did not correct the problem. Where is the voltage regulator. What does it look like? I assume it is some kind of Electronic Black Box. Engine is V8 5.9L

Often, they were separate boxes mounted on the firewall or inside fender. Here's what they look like:

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Follow the wires from the alternator.