First try the fuses that are hidden behind the cover to the left of the Lockable Glove Box. There are several fuses in there and one of them is for the horn. It may be either blown or missing.

Otherwise just getting at the wiring is somewhat of a nightmare. Among other things the largest single piece of plastic on the bike has to be removed. To do that you have to take off the following: the handlebar covers; the part of the dash under the handle bar covers;  the surround around the rad; the centre piece on the front of the nose of the bike; peel back the floor board covers on both sides. Do not try this without the proper shop manual.

All of that takes two people about two hours. Then all you have to do is fix the wiring problem and then put all the bits back on.

Recommendation: if you’re going to do all of that anyway to fix the horn, buy a Stebel Air horn and install it while you have all that stuff off. The 650 horn is pretty poor for normal use.