Apply a direct 12V to the wire going to the dash lights. Bypass the headlight switch. You always isolate a suspected component to test. If they light good, it points to the rheostat and/or switch as suggested by Carl. If they don't shine any brighter, it points to the ground as Xena says.
We will take your word that you are positive they are 12V bulbs and that this is a fairly recent thing. Not, ever since 12V conversion.
Bulbs must have good ground contact inside socket, outside of socket needs good contact to dash holder/dash hole, dash holder to dash, dash to body, body to frame, frame to motor, motor to negative battery post.
Isolation meaning in this sense: nonparticipation in or withdrawal. Bypass it's function to isolate it as the bad actor.
