Pull the wire off the field spade to the voltage regulator. Throw the meter on the alt output post and ground.. and you'll see battery voltage. Now use a jumper wire, engine running, and touch it from alt output post to the field spade and the alternator should go full field and show you 16+ volts. Then you know you're looking at the feed wire from the battery/bulkhead connections to the voltage regulator as the culprit.