Ford Falcon: Fitting child restraints

WANTING to protect my grandchildren, I tried to fit a full harness and booster seat to the middle rear position in my ED Falcon. I couldn't shorten the anchor strap for the full harness because the fitted centre anchor bolt position on the parcel shelf was too close to the seat front. The recommended installation point is 20cm from the seat front and that is where it is located. They could be fitted on either side because the anchor point is farther back, but it is not in the safer centre position. The booster raises the lap part of the harness too high to allow correct adjustment of the attachment strap to the anchorage point. The anchor bolt cannot be repositioned farther back because of a cut-out for a sound system speaker. There is no warning in the instructions about the full harness/booster combination.