A tether may help. It may be the only way to keep a safety seat secure if belts are anchored forward of the seat back. A tether aids protection even when the safety seat is held firmly with the lap belt. Some manufacturers have tether kits for their forward-facing safety seats. Do not attempt to install a tether on a safety seat not made to use one.
Many vehicles have holes drilled behind the rear seat to hold a tether anchor.
Some have nuts installed. Check the owner's manual for tether anchor locations. This special clip is available only from Ford, Toyota, and Nissan dealers. Your vehicle owner's book may explain how to use it. Some are a little bigger, about three inches long.