bora passenger door lock

[mystic 8v]

hi,i have recently purchased a bora 2000 model year,when i lock the doors from the inside off the drivers door locking button,all the doors lock but then the front passenger door rod pops back up,it locks fine from the outside and stays locked,could it be the front passenger door lock it self or maybe a central door locking ecu fault? where is the central locking ecu? please help regards stu.