I used a shop-vac with the crevace cleaning tool from my indoor vacuum cleaner attached to the shop-vacs hose and then connected it to the exhaust air connection on back of the shop-vac; this produced a high volume, narrow band of air, and the crevace tool was narrow enough, and the same shape as the drain inlet, to force that air through the drains. I was having the same problem with water getting into the engine compartment and this worked great for removing the blockage i.e. leaves, dirt, twigs and such. After, I checked by running water down the windshield with a garden hose and had excellent flow from the drains behind the front tires, you may need to reach under and "help" remove anything that might be to big to make it all the way out of the drain with your fingers, but otherwise I had no problems and it only took a couple of minutes, though you may have to be creative with the attachments, they're not always compatable; I had to add a few layers of black tape to the crevace tool to seat it in the shop-vac hose securely.