in the 2003 mustang manual it's on page 76
for those who are lazy here are the instructions
1 turn on the ignition swith to the run or on position do not start engine
2 wait until the safety belt warning light turn off.
steps 3-5 must be completed within 60 seconds
3 buckle then unbuckle the seat belt three times ending with the seatbelt unbuckled
4 turn on the headlamps turn off the headlamps
5 buckle and unbuckle the seat belt three times ending with the seatbelt unbuckled
6 within seven seconds of the safety belt warning light turning off buckle and then unbuckle the safety belt
7 confirmation that this worked the safety belt warning light will flash four times per second for three seconds
Just a note when your buckling and unbuckling the seat belt make sure to let it retract back each time.