The whole bumper must be removed from the car and be disassembled. Each bumper shock is attached to the bumper by two large brackets from the reverse side. I think each bracket is held by two 13mm (wrench size) nuts. The "push guards" are held by a bolt square in the center at the back. It can't be seen till the bumper is removed from the shocks.
The whole bumper will have to be diassembled. It is straight forward once off the car/shocks.
Bosch Master, ASE Master, L1
33 years MB technician