Hi msrdude
There are 2 parking assist sensors(ultrasonic). One left and one right. You must remove the rear bumper cover to get to them. There could also be a fault with the other sensor in your bumper that works with the park assist feature. This is called the radar sensor. The new park assist system uses the the two ultrasonic parking assist sensors and the hidden radar sensor to improve accuracy and range over existing ultrasonic-only systems. The radar is effective for sweeping the area immediately behind the vehicle and the ultrasonic sensors provide an effective range of up to 20 feet (6.1 meters).
Here are some diagrams for you to review that may very well come in handy for you:
PARKING AID SENSOR
REMOVAL
All sensors- Remove the rear bumper cover.
Ultrasonic sensor

- Remove the parking aid sensor.
- Disconnect the electrical connector.
- Release the locking tabs
Radar sensor

- Remove the foam absorber from the bumper cover.

- Remove the radar sensor.
- Remove the fasteners.
- Disconnect the electrical connector.
INSTALLATION
All sensors- To install, reverse the removal procedure.
- PERFORM Park Aid Calibration and Self-Test Diagnostics.
Any one of these sensors can be had for less than $100 online. The key is finding out which one is bad. Hopefully it's as easy as checking for the "clicking" as rtg says. Installing a new parking aid sensor shouldn't be too difficult. It looks pretty straightforward as you can see in the diagrams. After reviewing the diagnostic test procedures, I noticed that an ohmmeter is required for the troubleshooting process. Hopefully rtg is on track here though and you don't have to go through the testing procedure. Checking for a clicking noise sure seems a whole lot easier than probing a bunch of connections to get resistance readings. But if it comes to that, we can help you tackle that procedure if need be.
Good Luck!