Interior Light Door Sensor

Hi All,

The front interior courtesy light does not activate when any of the side doors are opened on my 1994 300GEL LWB.

However both light can be manually switched on at the light unit and the handbook indicates the switch should be at the bottom setting for the door sensors to activate the light when the doors are opened.

The back door sensor activates the back light ok.

As the lights work manually I presume the bulbs and fuse can be ruled out.

What I would like to know is there a common fault? 

If one sensor is faulty does that stop all sensors working?

Any suggestions would be much appreciated as it would be nice to have some automatic interior light.

Cheers

Martin