I fitted a Maplin kit shortly after getting the car. Put the sensors in a row down the middle of the bumper to be sure that there were no issues with scattering off nearby parts of the car. A friend decided to go all neat with his installation and put the sensors under the bumper so the back face of the bumper was a little behind the sensor. To his surprise and my non surprise the darn things saw the bumper and just wailed all the time. Ended up making little pods for them with short shielding lips. Many of the cheap aftermarket sensors are partially recessed to avoid similar problems.
Naturally another mate did a similar just below and behind the bumper installation with a different kit on a different vehicle with no problems.
I ran the sensor wires up through the long tailed "grommet" running up through the floor close to the right hand rear lights. Its a bit of a squeeze and you have to take the grommet out so it hangs down below the car. I taped my wires together in a staggered arrangement so only one plug was going through the grommet at a time. Put one section of my glass fibre rod cable puller thingies through first, taped the end of the wire bundle to it and just pulled them through. I reckon the larger diameter of the glass fibre rod as compared to string or wire helps open the grommet so the first plug goes through easier. Waving 3 ft of rod around under the car got interesting!
Best to get the back end nicely up in the air for this job. Theoretically it could be done by just pumping the suspension up into high mode but I reckon you'd need more than the standard complement of joints in your arms to manipulate things.
My control box lives in the rear light access area. Added a teeny toggle switch on a small bracket so I can turn the beast off if it has a meltdown.