I just used our regular, forward-facing child seat in mine. As you probably found out already, the rear seatbelts are bass-ackwards and a pain to fasten and the space between the tunnel hump and the side of the rear seat area doesn't allow the seat to sit down into the same area as your butt normally would. To solve this, I just put the seat on top of a thick bath towel, folded up to take up the space as well as protect your seat. Also, check places like Wal-Mart, they have a neat little thing that's legal in California that I may try. It's a "portable" car seat intended for vacations or taxi rides. It's a plastic back that fastens to a lap belt, then the belts mounted on the back pad hold your child in place. You didn't mention how old your child is, but above 40lbs, Ca allows booster seats that should fit a fastback.
