S.S.,

You are either running "on the Battery" or the "ALT" is not producing power

[SAME THING]
,
(for whatever reason: poor ground(s) , poor + connections , bad regulator)

Start with the basics, CONTINUITY on your cables.Then connections
Ozzy is putting you on the TRACK!
You have a Multimeter, So the rest is A.B.C.'s


Good Luck!