1990 325i interior water leak

Clean out the sunroof drains. Be careful as it's fairly easy to push the tubes out completely.

There are drain tubes just in front of the windshield that are intended to drain the water that goes down through all those louvers just behind the hood. These tubes can become clogged with many years of maple seeds, dirt and pollen. This turns the fresh air bay for the heater into a little reservoir from where the water can get into the heater plumbing that leads to the air vents just above the kick panel. Next time it rains pop the covers off from this area to see if the water is building up. The drains are rubber tubes about and inch and a half across and are flattened on the bottom like a noise maker. That's why they get plugged up. You can clean them out with a coat hanger or a small piece of dowel.

Check the scuttle drain is clear. Any water that enters through the grills next to the windscreen wipers collects in the scuttle and goes out via a black plastic drain tube.

Could also be a badly sealed windscreen.