Leaks in rain

My 1992 Corsica leaks in the rain. The floor of the back seat gets really wet, but I can't figure out where it is coming in from. Anyone else with this problem?

It might be from the door leaking. Maybe the seal or something in the door. I've been trying to brainstorm ways to fix a friend's car who's passenger door leaks while raining. It's not very fun to be sitting there as the door leaks on your lap or down the door and onto the floor.