The water is probably coming round the speaker in the rear door and bypassing the plastic sealing sheet and therefore getting inside the rubber door cill gasket and into the car. There are already drain holes in the bottom of the door so check these are not blocked. Drilling further holes will not solve the problem and create bare metal to rust. Neighbour had this problem and solution was to remove the door card and find where the seal had failed - around the speaker is most common and clean, reseal with 'gunge' and replace the plastic sheet.