Actually, just seen this, wonder if it'll work out ok? I guess it probably depends on the car, for the ones who've said it was great and those who didn't?
CLS63 SB, ML63, CLK350 'Vert, Triumph Sprint (Bike not Dolly...),
I just happen to have the underseat Isofix brackets for a W211 passenger side rear on my desk... I bought them for an early E500 estate, in the belief that the mounting holes and welded nuts beneath on the body shell were already there on the shell, but it turned out they were not....
I never got round to returning them. The part numbers are A 211 860 03 87 and A211 860 07 87. PM me if you would like them?