Sometimes it is hard to find. Often the cowl seal is crud and isn't really doing its job. Sometimes the metal under it rusts out and this adds to the problem. With my YJ, I used a hose as Cakes mentioned above, but I took off the spray nozzle first and just let the hose empty at low pressure. If I found no leakage, then it was nozzle time. When doing this, don't be haphazard. Spray one area and then check thoroughly before spraying another. I am almost tempted to scrape it all off and fill it all in with windshield sealer. On my YJ, I have a custom dashboard and ran my windshield wiper motor's wires different than OEM. So I can access them if I have to.