One problem related to child seat tether strap has been reported for the 1999 Ford E-150. The most recently reported issues are listed below.
Cosco convertible car seat manufactured in 02-04-00; #12935mag. Impossible to tighten harness around child when in rear-facing position. 3 inches of slack in harness webbing at shoulders. Contacted cosco, they deny problem. Cosco said it is my vehicle. This happens even if you try to do it on living room floor. I contacted NHTSA via phone to find out the standard for max amount of force allowed to tighten belts. Was told someone would call back. No one did.