James, most likely a dried up filter or bypass capacitor.
60s solid state and semi-solid state radios don't use vibrators to develop the high voltage AC; the transistors were designed to run at the low DC voltage provided right form the car battery; new type tubes were also developed in the late 50s that ran at low plate voltage as well.
James, The radio in your '55 uses a 6 volt 4 pin vibrator; however, if you're going to replace the vibrator you also need to change the buffer capacitor underneath the rectifier socket. Yours uses a .007MFD 1600V cap. Otherwise, the new vibrator will be damaged.
