Wonky rear ceiling courtesy lights

[GH77]

My rear ceiling courtesy lights don't come on either when the rear doors are opened or when they are switched to '1'. The front are fine.

They do come on, however, when the front ceiling courtesy lights are manually switched to '1' and the rear are also switched to '1'. Suggests to me that the fault cannot be a fuse as they do actually work, just not as they should.

They don't come on with any combination of front doors open/closed and/or rear doors open/closed. The rear outer reading lights come on okay when pressed, it's just the inners which don't.

All footwell lights are working just as they should.

Can't think of any other relevant information to add . . . . .

Any 'where-to-start' suggestions, please?