this was on the classic design concepts website
NOTE: 1999 & UP Mustang Bulletin
Due to the larger size of the seat belt shoulder loop on the 1999 Mustang
, modifications to this loop is necessary or the purchase of new loop is recommended Seat Belt retainer bracket PART # (Left) - F4ZZ 76610C07 A (Right)f4ZZ 76610CO6 A
Lift headrest and headrest support bars trim all the way up.
Insert wood stick (or slim jim) down the front side of the headrest bar. Push stick down as far as possible . When the stick is beneath the headrest trim piece, and push headrest all of the way down.
WHAT DO THEY MEAN BY THIS?
Bend seat belt shoulder strap loop down so that the leg that attaches under headrest is flat
Remove seatbelt loop and torx bit.
With template shown below, center punch for new holes and mark for cut off.
Drill 1/8" Pilot holes
Drill 13/32" hole in plastic only (on topside only)
Drill 19/64" hole through remaining material
Cut off end on new cut off line mark
Clean off any burs and re-install