Is this a 12 volt radio? (I am guessing it is, standard dash radio type radio)
Your batteries are either 1: Disconnected or 2: Toast.
There are several differnet converters.. One popular one in days gone past in low end rigs was the Magnetek 6300 series, converter/power distribution center (Check the door of your fuse/breaker box) This unit has very poor regulation (Feroresonant transformer) and no filtering.. It relied on the battery to filter the DC to the radio. It also boils batteries dry fairly quickly.. when dry they no longer filter.. And when they no longer filter you get a very loud buzz and teh radio no longer works.
Now magnatek is no more (parrlex bought them) But they did sell a "Battery Simulator".. Did, past tense.. If you go to a high end auto audio store they have somethign called a "Hardening condensor" or "Hardening Capacitor" (Two names, same device, make that 3 names) It is,, the same thing as the Magnetek Battery Simulator.
Home is where I park it!