What do you need to do to get your charging system to work after installing a instrument cluster with tach in a 1993 Ford F150 5.0L?

They might have a different internal voltage regulator that is used.The idiot light in some systems acts as a resistance that makes the regulator to charge.try putting a small automotive bulb in series with regulator connection at the alternator and see if your gague reads any voltage then.Check a motor manual to define the correct connection post.