A 5/8" bulb, would be a G5. The 1157 style dual filament base is referred to as a DC bayonet base. DC in this case means dual contact.
The only G5 bulb / DC bayonet lamp I can find is a type 302, rated at 28 volts. Its an aircraft lamp.
Doug, are you sure about the size of the bulb? Would a 3/4" bulb fit? That's a G6 bulb, and there are several G6 / DC bayonet lamps rated at 13 volts: #90, #99, #68, #96, #1178. These are all automotive lamps.
your illuminating friend on the DTBB
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.