Middle is usually the safest, but if you cannot get a good install, and can, say behind the passenger seat, then put the child there. Honestly, you don't WANT her/him to hit anything in front of her/him. The backs of seats have no padding and are usually just metal and springs...something you don't want your child to hit in a collision. The seatbelt or
LATCH is what is holding the child back. That is why it is IMPERATIVE to have your seat properly installed. If there is too much movement of the seat, it could actually cause the seat to malfunction and not properly hold your child in.
DEFINITELY have your seat checked out by a certified technician!