Stupid Design

We took one of our company vehicles to Mercedes Centurion on the 18/02/2016.Not for the first time, the service we received was disappointing;The ML is the 4th Mercedes Benz we have purchased and if I am frank, might be our last.The day we went and collect the vehicle in 2012 the salesman neglected to tell us of a scratch on the bumper until after we signed.This was later finally repaired after we had to send the vehicle back twice.This also happened after we had to wait for two weeks, just to take delivery of the ML.Now between then and yesterday we have our fair share of issues with the ML, but I have now reached a point whereby I can't take it any longer.While on holiday we noticed that the park sensors on the vehicle no longer work. but we were told that it's not covered do to stone chips.We were also shown where the unit is located.I have a few issues here;1.How can normal day to day use case this unit to fail as all vehicles will have a life of chipping from the front.2.The placement of the unit is stupid if it is in fact so sensitive that a simple chip would damage it.3.The terms and conditions are extremely vague and in our attorney's opinion not enforceable either.