Have you already purchased your seat? I only ask because it may be worth a little extra to look into the LX version, or the LX base for that car if you have two vehicles and will be purchasing an extra base. It has a built in lock-off for the seatbelt, which means you would not need to use the locking clip. It makes things much easier!
ETA: Oh, also, does the rear middle seat have a shoulder belt or a lap belt only? If it is a lap belt only, it may have a locking latch plate on the buckle. The buckle piece on the seatbelt doesn't move freely, but locks into place when it is pulled tight. You should be able to install in that position without a locking clip or lock-off. Beware, though, because in our 1999 minivan, the locking latch plate doesn't seem to stay as tight as I am comfortable with, so I still use the lock-offs on our infant base as extra reassurance.