Bruno,
You will have to take the upholstery of the door to gain access to the switch and wiring. Try this site :
http://ow.no/mb/w124/tech/
http://ow.no/mb/w124/tech/electrical/centrallockingsystem.html
Unfortunately the removing of the upholstery-site is not available anymore.
But I hope this will help you out.
Good luck,
Danny