If the radio didn't work, you are going to have to dig everywhere the wire is and do an inspection with a multimeter every 6 inches or so along the wire, so you can pinpoint the break. Take all safety precautions of course, it depends on the system you have installed. Once you find where the continuity is broken, take it in smaller intervals until you can see the break in the wire/loop. As for fixing the wire, I'm not the best person to ask, as I will probably give you the wrong instructions given the type of wire. It is PROBABLY just like any other repair, but don't quote me on that.