The central locking on our 1999 L series 5d only works on the left hand side. There is no noise from either door on the right hand side when locking. The drivers door won't lock with the key either but will lock when pushing down the little plunger on the top of the door.

Where should I look first? Is there a common fault that occurs?

Our Td4 drivers door won't central lock either, but does still lock with the key, so I presume it is a fairly common problem.

I was thinking of pulling off the door cards to see if power is getting to the modules and chasing the loom back to check for any broken wires.