Not to a Doblo, but I did to my Focus.
It's a relatively easy job. The usual "measure twice, drill once" is pretty obvious - other than that:
- once drilled, use a small screwdriver to clean the holes - if you don't scrape them slightly you can fit the sensors (thin layer of melted plastic builds up).
- Having an extra pair of hands makes running the cables much, much easier.
- Don't worry about hiding the buzzer...they are loud enough to be heard
I'm not sure how or where you can route the cables on a Doblo. The Focus has lights mounted on the rear bumper, so it's easy enough to thread the cables through the existing grommit