I can't really diagnose your vehicle, but I can give you some info that might help you. (Just ignore if not interested.)
Your vehicle has airbags.
Disconnect the battery negative cable before replacing any parts.
from the 2007 Factory Service Manual (FSM):
A single note (domestic) or dual note (export) electric horn system is installed on this vehicle.
The horn system features electromagnetic horn(s). The horn system includes the following major components:
Cab Compartment Node CCN) - The CCN integral to the instrument cluster.
Horn - The horn(s) are located on the left and right side frame rail.
Horn Switch - The horn switch is located under the driver airbag.
Total Integrated Power Module (TIPM) - The TIPM is located in the left front portion of the engine compartment
The horn system operates on battery current received through the Totally Integrated Power Module (TIPM). A signal is received by the TIPM from the Cab Compartment Node (CCN) over the Controller Area Network (CAN) Bus when the horn switch is pressed, and the horn(s) sound(s). The horn system circuit is designed so that the system will remain operational, regardless of the ignition switch position.
Diagnosis and Testing
HORN DOES NOT SOUND
CONDITION - POSSIBLE CAUSE
1. Check fuse 23 in the tipm. 1. Replace fuse if inoperative.
2. No voltage at horn relay terminals 62 AND 66 and fuse is ok. 2. No voltage, repair TIPM as necessary.
3. Open circuit from terminal 65 of the relay to horn switch x3 circuit. 3. Repair circuit as necessary.
4. Inoperative or damaged horn. 4. Voltage at horn when horn switch is pressed, replace horn.
5. Inoperative horn switch 5. Replace horn switch.
DIAGNOSIS AND TESTING - HORN
1. Disconnect wire connector at horn.
2. Using a voltmeter, connect one lead to ground terminal and the other lead to the positive wire terminal.
3. Depress the horn switch, battery voltage should be present.
4. If no voltage,(Refer to 08 - Electrical/8H - Horn - Diagnosis and Testing) . If voltage is OK, go to Step #5.
5. Using ohmmeter, test ground wire for continuity to ground.
6. If no ground repair as necessary.
7. If wires test OK and horn does not sound, replace horn.