Antpet,
Sure does sound like a short to ground somewhere in the circuit. I suggest you get the wiring diagram for your 2002 Xterra and begin the troubleshooting with it. You should be able to to find any common circuit elements between the radio and rear lights. If not, your Dad may have just busted up some wires connected to the rear light circuits while fooling with the radio. I got the diagrams for my 2000 Xterra from
www.ahdol.com. They're inexpensive and download in convenient pdf format. The other good thing about these diagrams is that the wire insulation colors are identified so you can trace any rear light circuit wires that my be passing in the vicinity of the radio.
Good Luck,
Geezerman