I fitted mine (4 sensor type) with the bumper in place. It was easy enough.
HOWEVER!! in use, these sensors are intended to be angled slightly upwards, something not mentioned in the instructions... In fact some kits now come with angled washers to accomplish this - mine didn't. As a result I fitted mine facing straight back - this makes them useless as they pick up the road surface and give a continual warning sound.
Now they're permanently disconnected.
Also be aware that the upper area of the bumper has expanded foam stuck behind it for impact absorbency in shunts, this can make it tricky to find a decent point for installation.