have you had the windscreen out? take the stainless trims off around the windscreen and have a look its a lot easier if the screens out but if there are any holes in there which is common too then it will seep into the cab ive seen water basically run down the side and behind the kick panel if there is rust at the corner of the screen, id take the stainless off and maybe on the driver side look behind the kick panel it may be running down in there and thats why you cant see it on the firewall