How do I stop rain water from leaking in to the cabin?

The motion for the wipers comes from machinery inside, under the dash. That small post that the wiper arm hooks to may be loose, allowing rain in. Take off the wiper arm and look for a nut to tighten

Do you have a sunroof? If you do, the drain tube may be clogged and water is seeping into the cabin. The tubes can be cleaned out with compressed air.