I had a similar problem last year with my 2002 - the entire back end went dark. No lights, brake, signal, running, etc... I didn't try the horn, so I am not sure it that was part of this problem or not.
It ended up being a ground wire issue. Since I was on a trip at the time, we were not able to trace it completely, but I was able to add a new ground via one tail light and that seemed to work. Or it has since June 2011.
I have seen some comments on various forums that losing the ground wire is a common problem with the Wings.