I own a vintage (1968) automobile, equipped with a vintage radio. The previous owner installed slightly more modern speakers. The radio works, adequately. However, it is not able (of course) to accept my mp3 player as a signal source. As I stare at the pinout data of a USB connector methinks I could cobble a few devices together, splice it into the connections for the radio, and it would work, while drawing the 12 VDC current from the battery for power. (I like the look of the old radio; yes it would be easy to install a modern radio with the USB connector, but that is not what I want...) However, I do not understand exactly the D- and D+ signal lines from the USB; are these considered line level sources? Also, I probably would need some type of step down device to convert the 12 VDC to 5 VDC?
Thank you in advance for any and all ideas!