I have Googled the cr*p out looking for this information to no avail. Has someone the definitive answer ? Not under spare tire, not in PS footwell, et. al. Much appreciated in advance.

Found it, on my 2009 C300....trunk, driver's side inner fender, behind liner flap near top, adjacent to hinge.

I’ve been looking for mine, and was shocked to find it is not in the spare tire well. Mine is right where you said it would be.