Your friend is right, there are two fuses that are needed for the radio, but the empty slot you found in your fuse box isn't where one of them goes. Confused? Here's what the fuse panel looks like:

The
rectangular RADIO Fuse with the number 10 in it is where one of them goes. On the other hand, the
square box near the lower left hand corner marked Radio, is only an outlet that other devices can be plugged into. That outlet is fed by the Radio Fuse so that's why it's marked that way. The second fuse that powers the radio is actually the rectangular one marked BATT, above and to the left of the Radio Fuse. It's a 20 Amp fuse. Once again, there are several square outlets on the fuse panel that are also marked BATT, but they are for other accessories to plug into, in order to draw power from the BATT Fuse.
[This message has been edited by Bloozberry (edited 11-20-2011).]