actually i had the same issue took year to finally locate the problem
u see line between hood and wind screen where wipers r installed
water enters from there and its was very small holes on left and right side under fenders to get out
when they get blocked ...water fills up there and rust the fire wall.....and after some time u get water coming out of there
ehrn it rains alot of water enters..during car wash water is not in that much quantity which can move into yr car shell
before dash board..remove fenders and see what is the condition of the fire wall