How can I fix my ford transit van central locking? ?

My 2005 transit 300 central locking has a mind of its own. Sometimes locks fine and other times I come back to van after a couple hours and it's sitting there unlocked. Sometimes it unlocks as I'm walking away. It's very random. I've tried double locking and manual licking but there's no key hole on passenger door. Btw, it's only the cab doors that unlock.one thing I've noticed is the corner indicator lights no longer flash to let me know the locking system is activated.
Any thoughts? I'm camping in Spain so all my possessions are in there, except I take passport and cash with me. Please help