I used to have a 107 with a jumpseat. I used this for my young kids and it was convenient.
I now have 129 and I purchased a jumpseat for it. With the top on, headroom is limited. Though I wouldn't be without one, as it's great for that "surprise" passenger that can, at least, have a decent upholstered seat to be in. Leg room and headroom are questionable, but not bad for a 4 seat sports car. I have a Porsche 911 that has factory rear seats w/belts that are smaller than the rear seat in my SL, and I was able to take 4 adults along, in that car. Albeit that they were NOT 6 footers nor 200+ pound individuals. You do have restrictions . . . but in a pinch it's better than the luggage rack or a long walk in the night.

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