lights problem

My father has a 2008 90 and in the last couple of weeks he said his interior lights have stopped working when he opens his back door but all come on when he opens his front door. I have got him to check all fuses and are all fine he says his switch seems to work and move. Im heading up to his this weekend has anyone had this problem to give me pointers of were best to start to speed up the job that he has donated to me so I can get on with the long list of other jobs he wants me to do

the switch in the back door that brings the light on may be faulty or a wire off. needs checking

Will have a look there first. Just thought Id ask first just incase its something that happens often before i start with the multimetre and removing bits.