In reply to Enty:
If you have a Ford Focus you are in luck - I used to, and this works....
Mk1/2(2000-2011)
Permanent deactivation: Before starting, make sure that:
The parking brake is set.
The car is in Park(automatic), Neutral(manual).
The ignition is OFF.
All doors are closed.
The driver's safety belt is un-buckled.
The parking and headlights are OFF.
1) Turn the ignition switch to ON, but DO NOT START.
2) Wait for the safety belt warning light to go OFF(1-2 minutes).
Steps 3-5 MUST be completed within 60 seconds!
3) Buckle, then un-buckle the driver's safety belt 3 times, ending with the belt un-buckled.
4) Turn on the headlights, turn off the headlights.
5) Buckle, then un-buckle the driver's safety belt 3 times, ending with the belt un-buckled.
After step 5, the Safety Belt Warning Light will be turned on for 3 seconds.
6) Within 7 seconds of the safety belt warning light turning OFF, buckle then un-buckle the safety belt. This will disable the Belt Minder if it is currently enabled, or enable the Belt Minder if it is currently disabled.
Confirmation of the disabling of the Belt Minder is provided by flashing the safety belt warning light 4 times per seconds for 3 seconds. Confirmation of the enabling of the Belt Minder is provided by flashing the safety belt warning light 4 times per seconds for 3 seconds, followed by 3 seconds with the safety belt light off, then followed by flashing the safety belt light 4 times per second for 3 seconds again. After confirmation, the deactivation/activation of the Belt Minder procedure is complete.
MkIII (2012 and newer)
On the new Focus, each front seat must have the belt minder disabled separately. For the passenger side, it's best to have a friend who's willing to help. The below procedure will toggle Belt-Minder functionality for whichever seat you wish.
Turn the ignition on (RUN, not start)
Wait about a minute for the seat belt light to turn off on the instrument cluster.
Buckle, then unbuckle the seatbelt (either the driver or passenger side), 4 times, ending with the belt unbuckled.
The seatbelt light will flash 3 times
Within a few seconds, buckle then unbuckle the seatbelt once more.