If you're still 6 volts like I am you may be interested in what I did.
As I was considering a sound system as an alternate to my AM radio I thought of doing what you're thinking but that would have required a 6 to 12 volt converter for about $100 plus the rest of the system. Instead, I just bought a headphone amp for about $100 that ran on a 9 volt battery then hooked it up to my radio speaker through a switch mounted in the roof of the glove box that I could switch the speaker source between my radio and the headphone amp and pluged the headphone amp into my DiscMan CD player and put it all in the glove box. I soon discovered, however, that running the speaker on the headphone amp soon killed the little 9 volt battery so I bypassed that and hooked the little amp up to a 9 volt battery pack I made out of 6 D cell batteries in a Radio Shack holder as shown. It lasts for a year or two. Because the single radio speaker is monoral rather than stereo as comes out of the CD player and goes through the amp I added a Radio Shack adapter wire that goes in stereo one end and combines both tracks into one monoral signal coming out the other end for the speaker. The system was somehow picking up the spark plugs so I got the choke coils from Radio Shack and put on all the wires to eliminate that. So, basically for the same as I would have paid for the 6 to 12 volt converter alone I had at least a CD player that played through my radio speaker. Whenever I take guests for a ride I put on 40's music and they wonder what radio station I'm playing that old music from. And, at car shows I run the 40's music out of the radio speaker all day and the neighbors and visitors love it.
I have since replaced the CD player with an MP3 player that is also an AM/FM stereo radio receiver.
Just an idea.
