We have a c3 picasso, with a rear facing seat on the passenger side for a 3 year old (with a block to deal with the floor storage problem). Plenty of room for the passenger.

We spent a few weeks in Aus/NZ and bought a couple of car seats there (cheaper than renting them), which was odd -- they were rear facing and belted, but had straps to go over the back of the passenger seat and connect to an anchor point in the boot (trunk).