When I was kid, I bought an FM converter for my '51 Ford. It ran on 6 volts and you unplugged the antenna from the radio, plugged it into this thing which had an antenna-like lead coming out of it that plugged into the radio where the antenna usually went. In theory, you tuned the AM radio to an unused station (i seem to remember it was 1440) and you were then supposed to be able to listen to FM stations using the tuning knob on the device. I don't think it worked very well, as I don't remember much more about it, other than installing it.