I use the jumpseat in my R129 quite often. It is fitted with two lap belts. I think it is homologated as a four seater. Certainly it was marketed here as a 2+2. The rear seat was a standard fit item in the UK. I have had two adult women in the back, but it was a tight squeeze and just a 20 mile journey. My 14 year old sister rides in the back quite a lot (she is adult sized); she sits lengthways with her feet on one side of the seat. I can't see any problem putting tots and small children in there. My brother has squeezed in there a few times, too, he is 6ft plus.

(It is best to put the roll bar in the up position when riding with people in the back seat, lest it pop up and take your passengers' heads off.)

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JJ Rodger
2013 G350 Bluetec
1999 SL 500
1993 E300 diesel T
1990 190