I tried to use the horn this morning when someone cut me off pulling out of a gas station but it failed. When I got to the office I pulled the 7.5a and 20a (since I have the Marco Tornado air horn) fuses and both were ok. My next thought was that I may have bumped a connection when I installed the CM Spyder Grill. So I got down and looked through the grill and found the horn laying on the plastic liner?! It looks like the main horn bracket rusted in half as part of it, with the bolt, is still attached to the body. Guessing that when it fell, a connection came loose resulting in the failure this morning. At least I found it now as I have it scheduled for a state inspection this Friday.
*edit - confirmed that the bracket rusted in half