Regardless of what anyone meant here's what I suggest the original poster do. The horn activates with an application of a ground from the button. The mute button for the radio is on the steering wheel/column so therefore he should concentrate in that specific area even if it requires pulling stuff apart. Based on his symptoms it is quite obvious that there is either a wiring or component short between the radio mute and horn circuit. The circuit board panel holding all the buttons on the steering wheel would be the most likely suspect. There are isolation diodes and one could be shorted or there could a shorted printed circuit run. BTW - A dead short to ground should pop a fuse someplace. Just my 2 cents.
Thrasher CAI, DHP v1.0 ECU, Granatelli CAI Tuneable MAF,
3.4 SC pulley, Hi-Flow CAT, u-bend delete, 180 TS, TB spacer
ET: 14.3 @ 96 mph