aftermarket parking sensors

seen a kit on micksgarage for less than 40 euro and was thinking of buying as im fitting my towbar this week(finally) and would be easier with bumper off.anyway is there factory holes in the metal behind the bumper or does it have to be drilled.be a bitch to drill holes if some were there.fairly sure I heard of someone on here or other site on about similar but cant find the thread.car is e39

On the E90 there are markers alright on the inside.
The E39 has 2 in the bumper and 2 in the rub strips so may be a different set up.

http://www.bmwhaus.ie/forum/showthread.php?tid=794