Often, leaks from rain that show up under the dash are from leaky
windshield seals.  Don't overlook the fact that antenna mountings can
also leak causing wet carpet on the driver's side.

While you're checking out its water tight integrity, check all the
grommets where wires run through the front to the headlight area, too.

You'll find postings in the archives where some have gone around (under)
the windshield seal with silicone with success.  I suppose it depends on
the overall condition of the rubber .... and the wallet.

Mike