Goes from the inside firewall to the heater core in the passenger compartment. There will be 2 metal tubes going through the firewall which will have rubber hoses connecting them to the core.

One other thing is it could also be your windshield mating surfaces in that same area. This happened on my Gen I. There was rust and it had gotten bad enough that the rubber seal no longer worked to keep the water out because the rust particles effectively pushed the seal away from the metal surface and created a big enough gap that water would seep in when driving (wind pushed the water up to the seal) but not when pouring water on it when it wasn't moving. <img src="/forums/images/graemlins/frown.gif" alt="" />