1971 alternated brown wire only has 4.4 volts

[Big 71 Jimmy]

New to the forum 1971 Gmc Jimmy running 350 with external regulator, battery not charging checked alternater not charging jumped F and 3 on external regulater, then alternator charged at 16.5 volts. Changed ext regulater. No help. Found brown wire coming from switch supposed to be at battery volts (12 volts ) and it is at the key switch but when the wire gets to the regulator I only have 4.4 volts not sure where I am loosing voltage at. Also found my circuit board on back of gauges going to my ammeter was fried, so I have ordered one of these. Anybody have any idea what I should do next?