There have been a number of issues with these - though more usually with the 5 series touring - where leaks around the tailgate have caused water to pool into the battery compartment. I believe there was a BMW TSB which involved the removal of the grommet - which is then usually left in the battery compartment, so that anyone working in the area in future can see that the grommet has been taken out. Removal of the grommet allowed water to drain away.
Anyway. In your case you'd have to PROVE - virtually to courtroom standards - that the only place for water to access the area is via that grommet hole. Considering the problems with water getting through tailgate area is well known, that's not likely to work. Even if it was accepted ...
You'd then have to PROVE that the standard of the workmanship was very poor, and the fitter was not 'diligent' in his work. The simple fact that the sensors have worked perfectly for 3 years rather kicks that into touch.
Basically, no chance. Forget it.
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