Some time ago, when CBs' were the rage and every had one, I
bought a 6V to 12V converter from Radio Shack, who is now out
of business. I have it wired with a concealed toggle switch to
avoid battery drain when not in use. I wired a cigarette lighter base to it for a power source, and the converter and
lighter base are concealed under the front seat cushion until
needed. I also have a Garmin GPS, and the battery will last for 4 to 5 hours, but I just turn on the converter and plug in
for extended use. It was not expensive, and has been trouble free all these years!