Thanks to all of you for the tips.

Richard,
I'm not sure if the folding (or stationnary) rear seat will improve the safety (if we can still talk here of "safety"...). It surely improves the look but I have no budget for buying one right now ! In any case, I'm only planning to do very short trips in the country side and the belts should be enough.
BTW, if you've ordered the 4basic rear seat, you should have the schematics for the belts? The picture on the site seems to confirm that you should attach one end of the belt to the vertical screw behind the plastic moldings, then passing it through the molding by a pre-cut slot, the other end being attached to a hole in the middle of the back of the seat.
Thanks in advance if you can confirm.

Thomas