I began by pulling the carpet out of the right hand side of the boot just so it revealed the empty area the sat nav box would go (my car hasn't got one so its a good space, but even with the sat nav box in there should enough space for the parking sensors ECU and all the wires, below shows the space:

Then i added a bit of double sided sticky tape to the bottom of the parking sensor ECU just to get it stuck into place nicely:

With it stuck down with the tape I also used some cable ties to hold it down permanently, and to stop rattles or squeaks in the future:

I then had to get the power for the unit, that's a simple case of connecting the red wire to the reverse light, with the carpet off its possible to see the wires on the rear light, there is a plug going to the light, with wires on it, the wire to the far left is the positive of the reverse light (the purple one in the picture above), and then I connected the -VE to ground, using them blue clip connectors is the quickest method, then I connected the buzzer and the power to the sensors ECU, and also clamped the buzzer into place with cable ties:

Then I had to measure where the parking sensors would go, and finding a suitable location is the most important part of the job, now below shows where on the bumper the parking sensors fit best (there are arrows to make it easier to see the sensors):

This sort of line in the bumper is the ideal location for parking sensors because there is over 30mm of a flat surface allowing parking sensors (21mm diameter) to sit flush with the bumper for a tidy installation, and also more importantly there is no impact bars, or metalwork behind here so the parking sensors fit in fully without any trouble. To measure where to put the sensors is a bit tricky so I just got to chairs on each side of the car tied a string level between the chairs and that sorted the horizontal end of where the sensors would go, and I just measured from the ground with a tape measure where the sensors would go. But its entirely up to yourself how to measure where they go, and worth doing right, once they are measured I just used a screw to make a small dent in the bumper, and then I drilled the holes with the drill bit supplied with the sensors:

I then had to fit the spacers onto the parking sensors its what makes them go in the angle you want, without them the sensors would just point upwards in the gradient of the bumper, I used a metal ruler just to push the spacers into place tightly as they are a bit tricky to do it normally:

Here is one of the sensors in place showing the importance of the spacer at keeping the angle right (note this sensor wasn't fully pushed in hence the slight gap):

I then had to put the sensors into the right place, thankfully they are labelled by the manufacturer, from A to D, a goes on the left of the car, B goes to the Right, it was quite simple, then its a simple case of keeping the wires tucked up into the bumper and away from the hot exhaust, so a few cable ties were used, but next was the issue of getting the wires from the sensors into the car to the ECU, so I used the 21mm drill bit for the sensors to drill a hole into the boot beside the wire where the towing light goes in:

Then I was left with a hole more than sufficiently big to get the wires in, but i had to get some galvanising paint to stop it rusting and a grommet, so i went into the other shed to get them and came back to this:

The dog finally got into the car, had the car 3 months now and kept him out so far but today he got in, but i ignored that and painted the hole with some galvanising paint:


Then I sent the wires through and clipped the grommet into place:

I used a bit of silicone to seal this hole 100%, and with that done it was just a case of connecting the right sensor wires to the right plugs on the ECU, then i tucked all the wires away:

And that's it, I put the boot carpet back into place and its impossible to tell there is all the wires and ECU behind it:

I did the test of the sensors by putting it into reverse and driving towards a wall, they worked first time and take into account tow bars so there is no worries about that, here is them on the back of the car:

The parking sensors can be bought on ebay for less than £20, and they just take about 2 hours to put in and well worth the effort (much cheaper than it as an option when buying new), I know the sensors on my car are actually black but I am waiting for a can of spray paint in the post once it comes ill be able to paint the sensors to match, but even so they are barely noticeable at the moment. Hope this helps any of you trying to put parking sensors onto the back of your 406.