[Arran]

The power supply looks like it was off board connected to a female ended cable, which could have been connected to either sets of batteries, or a vibrator supply, the tube filaments may or may not have been in series. The earliest car radios were very much like a farm or portable radio installed in a car with a set of "A","B", and "C" batteries to power it, and almost all were aftermarket installations, Packard may have been one of the earlier marques to offer a radio as a factory option since they were higher end cars to start with. I could look in my Philco factory manual but I'm not sure if it has any car radio schematics in it, it was given to me by an uncle who tended to eschewcar radios and TV schematics so they may have been edited out of the binder.
Regards
Arran