Where do I start looking?

just in front of the windscreen under the bonnet is an area which houses the wiper motor.
This is below the windscreen and is designed to accept rainwater but allow it to drain away.
If the drain tubes are blocked with old leaves and dirt ect the water cannot drain and will flood into the passenger compartment.
tell tale signs are a sloshing sound on bends.
If no coolant is being lost ( that would point to a heater matrix leak ), then check the drain tubes.

gareth
November 2009

This is a repeat question - it has already been answered.

The taking out of the car seats is Your Call - you decide.

Where the water is getting in is an observational thing - the people here can't see your car.

Somebody has pointed you at the access hole for the aerial - presumably you checked this overnight.

When it's dried out take it to a car wash and look for the dribble.

Incontinance comes with old age - sad but true.

Peccavi
November 2009