If it's coming in through the door card, it can only be entering the door down the window.
Take the inner door card off (unscrew all the furniture then prise off the panel from it's pop studs) and have a look.
You should have a plastic liner
under the door card that prevents you from looking into the door internals.
If yours is missing, you can replace it by cutting the shape of the opening from a heavy-duty plastic sheet and "gluing" it to the edge of the door. The factory uses something like silicone sealant not miles different from black instant gasket to stick the liner to the edge of the hole.. but you can try silicone sealant or anything waterproof. Even Evo-stick will do.
And it smells nice...
Before you do that... grease up all the door lock mechanism, levers and joints, squirt some WD40 onto any electrical connector blocks (windows?) and lube the window runners, since you don't want to go back in there for a while.
Also check the door drains. The door has 2 or 3 slots at the bottom and yours may be blocked or welded over (if someone replaced the door-skin in the past). If you have no drain holes, the water will sit in the bottom of the door until it eventually floods. Water getting "into" the door isn't a problem.. it's designed like that... but it should all leave out of the drains.