my mkii mondeo 1.8 petrol used to retire a new battery every 2 years! i never got to the bottom of it! got wierd symptoms when it was playing up aswell. whentrying to start the car as the engine turne over slowly, the wipers used to sweep slowly even when they were turned off??? the first hint i ever got however was the central locking being really sluggish. ie unlocking one door after the other over about 4 seconds instead of almost instantaneously. there was obviously something wrong with the electrics but never got to the bottom of it before i got rid of it. u could disconnect the central locking as the module is above the drivers footwell and see if it makes a difference over a few days