12V systems on trucks started when, and how do i check?

[mark '87 930]

I have a '56 short box and i am wondering if it was ever a 6V system. I've read mixed articles where some say the 12V systems started in 1955 in chevy's 2nd series trucks and cars, but i want to be sure.

I'm rewiring the whole truck and my main concern are the wiper motor and fan. Were they 12 volts? When i took the wiring out, for some odd reason, the wiper motor was wired to the ballast resistor (reduced voltage would be my best guess).

My gauges all seemed to be running 12V from before, I'm just worried about burning out the fan motor and wiper motor.