Hello db welcome to YT! Steve@advance and
Buickandeere have good descriptions of what the
connections should be. The link Steve gave to the diagram
takes you to a multiple return search result so that is a little
confusing. I will direct link his top diagram. Your M will differ
if it in stock configuration because instead of a solenoid it
will just have a heavy contact switch that energizes the
starter, most are operated by a foot lever just to the left of
the brake pedals. So in the diagram place that switch in
place of the solenoid and you will not have the starter
button that would energize the solenoid. Main thing is as
was described, one side of the ammeter will go directly to
the post on the foot switch where the battery cable that
come from the positive cable connects and that will be the
only wire connecting on that side of the meter. The wording
for the ..solenoid wiring starter button.. is placed in that
diagram so it is a little confusing just disregard it and follow
the wire connections as shown. And also as pointed out the
starter motor amperage does not go through the ammeter.
If you make the connections and the gauge reacts
backwards as the alternator shows a discharge when
running and the light draw shows a charge you just need to
swap your wires side to side on the ammeter. Hope this
helps to clarify the answers to your question.
Generic 12 volt conversion diagram