There switches around the locks were only fitted for around 18 months, the others are switched from the motor itself, but these do pack up internally, they are not very good at all to be honest.
I already have replaced one of mine, but used a 4 door locking kit from Maplin's @£25, so still got three spare.
As said though just check for anything amiss, could even be a dodgy connector, at the central locking relay, not in the door at all.
tabetha