Do you happen to have a sunroof? There are drains running off sunroofs down the A and C pillars that can get clogged or burst due to age.
Check the underside of both front and rear doors on that side of the car for a small drain hole and stick some clothes hangar wire up thru them. Contrary to popular belief, the window seal where it slides down into the door is actually incapable of sealing- all the water simply runs down the outside of the glass collects in the bottom of the door, and is drained out the bottom on the outside of the rubber weatherstripping. When these drains get clogged the water backs up and flows inside the car.