Can't tell you which wire it is on an 01, but I'm willing to bet its a negative ground like the 05-07's. Meaning the circuit (horn) has power but doesn't honk until you apply the ground when you push the horn button on the steering wheel. If you can find that wire, you can tap it to provide power to your solenoid valve and tie in a switch to toggle between air horn and OEM horn. Which wire is it? I have NO idea, but the members of the12volt.com did.