Only thing I could think of was that the actual problem was in the original radio. If a short or overload there, it might have just overheated the wiring, with a failure at a high resistance spot whereever. might have been a tight binding, or a bend in the wire. Who knows? Glad you found it and got it fixed with no other damage. As you suspect, a power surge for some reason is a very good guess. I have seen nearby lighting cause a damaging surge, even without a direct hit. As the antenna wire was burned, that makes it even more suspect.
Noel and Betty Johnson (and Harry)2005 GulfStream Ultra Supreme, 1 Old grouch, 1 wonderful wife, and two silly poodles.