Looks like this was not a dealer installed option. Has anyone tried to install the 3rd seat from the E350 wagon?

 

Its been done a few times. Search should pull up the master thread. I think the last fellow had it completely done for him, so it was around $6k in new parts. Obv I think it could be done for far less if you sourced parks form a salvaged E350 wagon.

The parts all fit. Mercedes only didn't offer it because they felt it irresponsible to put kids backwards in a car that could accelerate/launch so aggressively, according to my resident AMG expert at the dealership (50% likely to be true).

 

You could make some extra money taking kids for a joyride at the carnival