Look in your DC compartment for one or both of those pink wires, folded back as spares. My compartment is the one directly behind the driver's side front wheel (the flooded-to-the-lip compartment). Likewise, the pink with black tracer should appear there as well as a folded back spare because that is where they would want to locate the control relay.
I'm guessing on the location, because as you see in the diagram, the wiring is all GS add-on. The relay solenoid power source arrives via Chevy workhorse AE-2129 wiring harness (the steering column horn button). Your pink wires at the under-dash connector (actually a plug for the switch I assume) enter the AE-2022 dash/distribution harness at the 15-pin connector & enter the AE-2174 Step Harness. This harness feeds other devices that are in the DC compartment; hence an assumption that your air horn wiring lands there. I observed probably a dozen unused, folded-back wires in my compartment. Fortunately, they are all above the high-water level.
