Bought another one...... this ones a '85

#3 Either bad ground between the regulator and firewall. or firewall and engine block.
Or regulator is bad
Or alternator is bad
Or a wire between these units is not connected either by a broken wire or bad connection.

My 81 dually with a 400 bb had no ground between the water pump housing and the block. Took me awhile to find that one.