`64 carradio

Hi people
I`m restoring (someone i know actually, a pro) my radio but need to find some more info about these radio`s. There are some questions left..

Actually i`m in search for a diagram for these radio`s, plymouth valiant `64.
What we want to fin out, are some parts we cannot link somehow.
The original voltage, it appears to be a 6 volt radio, but the car is 12.
Is the + on the chassis?
And some ather questions wich i`m not able to translate to English.
Hopefully some of you have a schedule or something, an can help me out,
Tnx!!