Central Door lock do not work at low temperatures ?

Ford Focus ZETEC 16V 1400ccm 4+1 door - central lock by key
At lower temperatures the key oppens only the driver door not the other doors. Sometimes after i lock the car next morning I find empty batery and the car do not start.
At temperatures higher than ca 20 degrees celsius all doors oppen normaly.
Took of the fuse and disabled central lock, but i would like to know why it is related on low temperature and how to fix the problem.
In the Ford service they told me to change all the locks

Gaetano
June 2009