Another option, check these guys out:
http://www.vintageautoradio.com/index.php?option=com_content&view=article&id=54&Itemid=64They can take your vintage 6 volt positive ground radio, convert it to am/fm with solid state components and additional inputs (hidden beneath or behind so it still appears original)...add new speakers and you're good to go. There is a caveat though. If you wish to have a 4 speaker system, a 6 volt system without an alternator can not keep up with that. You will need an extra battery in the trunk to power it (recharging on trickle charger between uses) or only have a two speaker system.
However, when I spoke to the owner about whether a 3 speaker system would fly with this setup from the main battery, he seemed confident that it could. Three speakers you say...what the hell you talkin' about pilgrim?
My idea was to mount two externally enclosed speakers on the package shelf (as I do not want to cut holes in the shelf for flush mount speakers) with a third in the front speaker grill. The two rear speakers are hooked up as normal l/r to the speaker A outputs. The front, center speaker is hooked up in what is known as a "Hafler Circuit"...the left and right positives of the speaker B channel are connected to this speaker, no negative involved. Everything that is common between the two channels is out of phase and cancels itself out...all you are left with coming from the front speaker are the differences (example...guitar, bass and drums are common to both channels, they cancel out...sax in the left channel only, flute in the right channel only, that's all that is reproduced from that front/center speaker).
The Hafler Circuit has also been referred to as "the poor man's quad", since this setup adds an element of depth to the sound, like you would get from a quad system, though usually left and right are the front speakers and the center speaker is in the rear. But, Vintage Audio can add a separate volume control (again, hidden underneath) for the B channel, allowing you to tailor the level from the front so it does not overpower the rear l/r channels.
For years I had my home stereo system hooked up like this, with the rear, single speaker on a separate amp so I could control its volume relative to the front speakers. It does not cause issues or damage to the amp(s) or the speakers...and it is an option for a 6 volt positive ground system in one of these vehicles.
Hope this helps in giving you additional options going forward.
Addendum...you can have 4 speakers in a hafler circuit...again, the positives from left and right go to the positive inputs of the two speakers, and a single line goes between the two negative poles of that speaker set.
