The Ultrasonic Rear Parking Assist (URPA --since everything in the XLR seems to have an acronym) system is the only electronic system that isn't connected to the XLR' diagnostic bus.
To troubleshoot it in any detail, you use the supplied MK-1, Mod 1 Eyeball and URPA LED indicators. During start-up, the LEDs are checked and if there's a problem, the red outside indicator flashes to flag a problem. To find out what the problem is, you need to install a jumper in the URPA module connector to short a pin to ground.
There isn't much to this system. A module, four sensors, an LED indicator, power, ground and a speed input. There is a chime control signal on pin 22 (Light Green wire) that the previous owner may have disabled.
Not knowing your abilities with a multi-meter, I'll pass on writing up detailed troubleshooting procedures and wait for more detailed questions.