I have a friend who works on lots of old cars, vintage and the like. one of his biggest problems is with water leaks. He suggests the following.
First offf, doors are not waterproof, they are not supposed to be. The rubber strip on the soor that seats against the window is for clearing rainwater off the window when it is wound down. Water goes down inside the door, that is why there are drain holes in the bottom of the door, make sure they are clear. If you remove the inner door trim you will find a sheet of plastic glued to the door panel. This is actually to keep the water away from the door trim and out of the car. Make sure that this is attaching to the door all the way around except at the bottom. He recommends that you leave the plastic inside the bottom of the door to shed any water back into the door. The rubber seal around the door is to keep out wind. If the plastic sheet is stuck to the inside of the door panel it will let water run down the trim panel and onto the carpet in the footwell. As someone has huggested, use a hose to test but with the door trim panel removed and the plastic sheeting in place. That way you will see where the water is going.
If door speakers have been fittd, make sure they do not penetrate the plastic. If repars are carried out on a door or lock, this plastic is often missing.