I wired a 9V in series with two AA batteries (1.5V each) to test out the truck‘s dash panel bulbs.
Turns out the bulb holder I tested first was faulty and 9V would have been enough. In fact, three of the nine bulb holders were corroded and unable to provide juice to the light bulbs. That explains why it was so hard to see the gauges at night!
To get by until new parts arrive, I was able to finesse some small pieces of wire in between the bulbs and the holders to get an electrical connection.