water from under dash has leaked onto the ecu and ruined

Water is leaking from under the dash onto the ecu and puddleling in the passenger footwell. It is not the heater core or A/C drain. It only leaks when it rains. The harder it rains the more it leaks. I have removed the wipers and plastic surrond and cleaned what apears to be drain holes at the corners of the firewall. The drain holes were blocked with leaves and dirt. Where is the water coming from? Leaking water has ruined the ecu which I have replaced. I need to stop the leak before it ruins the replacement ecu. I understand from a ecu rebuilder that this is a common problem with the vitara. Please help. Thanks