Horns go bad all the time, work on that last. Which radio, VHF?
You need to take a look at the back of the radio and find the power and ground wires, probably red and black. There is a chance that the red wire has an inline fuse along the wire run. Where is the radio, under the helm, flush mount at the helm or perhaps up top in a electronic box.. Based on where and how the radio is mounted, the folks on this site can give you more detailed instructions of what to look for.