My 2005 has the parking assist sensors on the back end, and the original HU-650. That said, it has never actually done anything on my car. Is there a button or some other thing that should make it go? Or a reverse sensor that may be hosed?
I don't really have an issue with the park assist on my car and its been very valuable in city parking situations. It will sometimes get fooled by rain on the sensors, but other than that its pretty good. As Hounddogger said if yours is functioning it will automatically come on and display "park assist active" on your stock headunit while in reverse. The park assist button mentioned above will turn it off, but it defaults to on as mentioned previously. Does your car have an AEM?