Probably best to start at the back !!
.
.
.
.
.
.
.
.
.
Depends on the car. My son has them on his FL1 TD4, but they never worked properly. When we had to replace the bumper, we cleaned up all the wiring. If I remeber correctly they worked a bit better, but only 3 out of 4 working. Generally they get full of water & road dirt, hence stop working (ultrasound doesn't go well through dirt !)