E30 Zone

hi ever since i canged the engine on my 1990 320i se for a m20 b25 from a m sport my battery dies over the course of a few days. if i charge the battery it will last a week then one morning i'll come out and nothing. i use the car every day it only takes 10 mins to get to work so its very short journeys. i've checked the alternator and battery and they are fine. just wondered if anyone has found a common drain on the battery. i also have no door locks and struggle breaking in through the boot.

any help much appriciated.