i had this in an old one of mine too.... the seal under the fuse box had perrished.

pull the carpet an underlay away from the rear of the pedals and surrounding area then get hose pipe or a jug of water and slowly let water run down the inner wing, best havin someone lookin inside the car. if not try lettin water run anywhere else you think it may come from.

only real way to find the leak is to get water goin thu eh ! it'll get a lil wet, but its wet anyway !