Why is my interia light on all the time on my VW passat?

I have a 1998 R reg VW passat & if the interia light switch is put into the door position it remains on all the time. I have had a mew door lock assembly fitted in the nsr door as that was thought to be the cause of the problem but it still remains. The boot light doesn't work either & the car has now a habit of locking itself. I have been told that its possibly the conveience unit under the passenger seat but before I fork out more cash I want to be certain.