i recently installed 4 sensors ultrasonic backup sensors on 2012 XC60. I am very happy with the result. Its very useful to me since I have a new neighbor driving a huge F150 next to me. it stick out so much. I have to back out carefully. Now I don't worry about hitting a car behind me.
I'll start beeping slowly around 1.5M and increase intensity to contiguous beep around 30CM.

To take off the rear bumper, there are 5 torx screws on each rear wheel well(need right angle screw driver head), 1 torx each at middle Top left and middle Top right of the trunk. Lastly, the hardest part is there are two plastic clip at the bottom. These two clips were secured by expanding by a round plastic rod in the middle. So to loose them, I used a small Philips screw driver head and push the middle rod outward from inside by wiggling a lot. After the middle rod stick out half CM, you can use a plier to pull out the rod and the clip is free. Pull left side and right side of rear bumper carefully, pull away from the clips and it'll become loose. Unplug the fog light connectors on both sides and the bumper can be removed.

Then you drill 4 holes on the bumper and install the sensor.Note the sensor has an upside. Before doing this, I tested the hole saw on a cardboard and the sensor fit perfectly.
Before you drill, make sure the there's enough space for the sensors. I tested the space by mouting sensors at different spot using double sided tape and the lower black portion is not suitable.Because the metal bumper stick out a bit. The top of the metal bumper is some room that where I drill the holes.

I chose the left side light power box to power the sensor so I run the wire to the left. Open the panel on the left side of the trunk. I found there's a unused hole with rubber protector near the light. There's no hole on the rubber so i made a hole. It must reserved from future back up camera installation. After doing research, I found backup sensor is more effective than backup camera in terms of accuracy and weather resistant.
After routing all 4 wires to the left and into the electrical control box area, put the bumper back to the car.

Next, find the reversing light wire. I found the official wiring diagram but it's very confusing because the same power signal wire can change colour at different section.
I used a volt meter and found the reversing wire. First, I found out which bundle of wire the reversing wire is located. Then I found the ground wire in the bundle by find any 12V pair and notice the reading is plus or minus. Then use the ground wire is a base point, I alternate the red probe at different spot and determine the reversing wire. (tips: activate emergency brake and the car won't move, then switch between P, R and test different wire. There are only 6 wires on the light wire bundle i disconnected. It doesn't take long to find it.

I don't bother to run the monitor wire inside the car panels because it's too much work. I just run it along the interior and stick the monitor at the bottom of the LCD display. It obstructs the LCD a little bit but not a big deal. The perfect way maybe run the wire along the ceiling secured by needle safety pin anchors and tape and stick the monitor on top of the rear view mirror.



You can get the 4 sensors system from Aliexpress for only 15 USD:
AOSHIKE Car Auto Parktronic LED Parking Sensor with 4 Sensors Reverse Backup Car Parking Radar Monitor Detector System Display