VW POLO Comfortline 2006. Problem with central locking?

The fuse for the central locking system is OK, and the two back doors lock & unlock OK with the driver side switch and the mobile remote. The boot door solenoid checks out OK, but the door lock doesn't operate, so I suspect a relay must be faulty. Where is the relay located? The boot door and the two back doors have until now operated simultaneously when the remote or manual switch is operated