The ceramic resistor, about 1 ohm, provides an equal voltage drop for a motor. 6 volts with about 10 amps current flow. Each gauge uses something less than 1 amp. With a ceramic the gauges are powered with about 9 volts. A 7806 3pin regulator wired to each gauge will provide a constant 6 volts from the 12 volt source.
This link is very informative:
http://www.studebaker-info.org/tech/6-12V/6-12-6.html