What's the 2015 GMC Sierra 2500HD horn location?

The 2015 GMC Sierra 2500 HD has it's horn under the hood in the engine compartment; it's usually on the driver's side, behind the bumper cover, below the headlights. The part is going to look like a round canister with a grill in front of it: You can see a schematic in the GM technical manual, part 84377259. If You're not getting any sound when You hit the horn, there's a relay You can check, in the engine fuse box. This is a common failure. If it's not that, You can also verify the wiring's okay, though that's not a common reason Your horn'll fail. You'd want to check the switch/latch in the steering column before You suspect the electrical system, beyond the fuse.