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.