On my GS, I used the support behind the fuse box on the left side. Take the tupperware off the left side, from the piece covering the stock horn, all the way back to the piece covering the battery, then remove the "Y" piece in front of the seat. Drill one hole in the small frame tab behind the fuse box, put the rlay and wiring harness in, positap into the existing horn wiring to power the relay, make sure it works, and button it up. If you've got all the parts, it should take less than half an hour.
john