I installed a kit from eBay onto my Leon, got black ones, and put them into the black strip on the bumper. Fits perfectly, and the access behind is good, once the bumper is taken off. I then took the 4 cables from the sensors into the car through the rubber grommet carrying the wiring for the number plate lights, and mounted the control box just under the nearside rear light cluster.
Only the black strip in the bumper is drilled, no metal behind or anything like that, the sensors just push into the holes made in the plastic. For extra security, i used silicon sealant to make sure no-one pulls them out easily.
Already at my limit of uploads, so can't post any pictures, but if you PM me with your email address I can send photos through.
Hope this helps
Mike