I think he might mean an AM/FM radio designed for marine use, as is to be mounted on a water going vessel of some sort.
If you still have the coax attached to the unused antenna, then an adapter should work if you can find one that fits. Most likely the antenna coax would have a PL259 connector, so an adapter to adapt that to whatever your AM/FM radio uses. I guess those use the old "motorola" connector.
I think SO-239 to Motorola male would be what you would be looking for, but since I do not know what radio you have (and if they have a special connector) then I cannot be 100% sure.
edit....
The AD259 on this page looks like it would be the one. I have not seen the back of any sort of AM/FM radio to know how deep the jack is, as to whether this will go deep enough to work right. I would guess so.
http://radioproshop.com/connectors-rf/connectors-rf.htm
I have never used that place before, just linking what I found with a picture and cheap prices :lol: