Power the newer radio directly from a small extra battery in the trunk?
Buy a portable self-contained Bluetooth speaker and send music to it from your phone?
There may still be 6v to 12v power converters available for this purpose but positive to negative ground could be a problem???
I use this in my 6 volt car, and it works great. I charge my phone, and use an FM to bluetooth adapter. It has a 10 amp limit, not sure if your CD player will use up that much power but for the price it might be worth a try.
Back when CB radios were the rage, I put one in my 1955 Thunderbird. I used an inverter and insulated the radio installation from the auto chassis by using a piece of wood, That way the CB (negative ground) and the car (positive ground) would not be in conflict. It worked great for years.