Did you change the horn yet...?
Did your car come with a FSM? That should give you a good idea of where any hidden clips happen to be.
As for replacing the horn, Pep-Boys caries a variety of air horns. You can probably find one that suites you and cleanley fits under the hood of your truck. Running the wire is nothing, just buy some extra vacume hose.
We just installed an airhorn in my friend's Toyota Tacoma, but the grill was attached to the hood so it was easy. They gave us like three feet of vacume hose for two trumpets.
Hella makes good, strong, horns. Their airhorns get up to 118dB and their standard ones get a little less then that.
This is a more compact solution, I've installed it with no extra hardware on some small cars before.