I have been using my stock 40 radio with a 12 volt system, and a voltage reducer for 25 years without any problems. Granted, you only get AM channels, but so what. I also mounted a tape deck and AM/FM radio under the seat. It's out of sight, and so if I want to play 50s and 60s tunes, or listen to FM radio, I can. I plan to upgrade to a CD player with
AM/FM this winter, replacing the old Tape Deck.
I got my votage reducer from Ron Francis Wiring, but I'm sure there are others out there for allot less money. Just be sure the voltage reducer will handle the amps the radio will need.
Dave