I never used to remove the bumper on these car's either, like dnsey says just be wary of where you position the sensors (2 or 4?). Will you be using a genuine Citroen system? with these you should get 2 types of 'angled' spacers to fit between then actual sensor & the bumper, i generally use the thicker of the two & angle the sensor upwards as they tend to be rather over sensitive & go off randomly

. If you are using the genuine citroen reverse sensors the holes you need to drill into the bumper are 19mm (i think), i used to use a 'hole cutter' but this tended to make a mess of the paintwork so ironically i always removed the bumper when using this & drilled it from inside, a regular 'cone cutter' will suffice but you may need to enlarge the hole if fitting a genuine citroen system.
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