How can I fix my Corsa 1.2L yr53?

Hi hope you can help I have a 1.2L Corsa and it gets wet in the front passenger footwell. The problem only happens when its raining and it appears to be coming in from behind the back of the glove compartment. The car hasn't had any modifications since new so no ones poked about threading new wires in etc....got any ideas where I should start? Thanks Lucy