Avensis 1.8 gls central locking

I have a Toyota 1.8 gls 5 door 1998, the central locking had gone weird, it will unlock but not lock, I have replaced the battery in the keys and tried both keys, is there a relay somewhere that could of gone, or can anyone point me in the right direction to cure this.

Thanks

GPW:banghead:

the relay contacts will need a clean to locate sit in car and listen for relay noise whilst operating remote