What is the brand and model of the radio?
Did the radio come with wiring instructions?

Most likely the memory draws very little current. There must be one connection to the radio for On/Off-power; and, another connection to the radio-memory that must be 12v all the time (to maintain the memory).

Is your 6v-to-12v converter switched-on by your ignition switch; or, is the converter always "on"?