Suzuki Vitara centrol locking - passenger door does not work?

hi,
my [friend's] Suzuki Vitara has central locking (but not remote activation)

when i put the key in the driver's door and turn, all locks will lock/open - except the passenger door. it twiddles a little bit but does not open.

when i put the key in the passengers door, all locks lock/open as they are supposed to!

so it seems the fault is with the central locking controlling the passenger door. is that right? what are the possible faults (and where is the central locking unit)?

it's quite annoying (and a little dangerous too, because you can 'think' you have locked the car - but the passenger door will still be unlocked!)

any ideas?
thanks.