So I just found this instructable online, and I bet one of you guys could make something of it.
A few weeks ago I remember se7en or twist or someone talkin about having a nozzle out the back bumper of your car with a remote switch to squirt people. Well guess what I found… 
http://www.youtube.com/watch?v=P8dDrCxFFbY
http://www.instructables.com/id/Remote-Controlled-Hose-Valve/?ALLSTEPS
none of that is really possible useing any of that on your car… the wireless part is a “remote controled switch” that runs on 110v he has a transformer that is hooked to a sprinkler valve which is connected to his home water supply…
so in other words when he pushes the remote button it turns the remote switch on that sends out 110v that powers the transformer which probably puts out around 12v that turns the sprinkler valve open that lets the already pressurized water from your house through the valve…
you really want to do this? do it right…
check these out… its an electric sprayer really all you would need to do is hook a switch up to it to turn it on and off and possubly modify the sprayer a little bit…
http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200347991_200347991
and would probably end up costing less than trying to build your own…
its 16 gallon tank (you may be able to find one with a smaller tank. 12 volt electric water pump 2.2 gallpns per minute… and the nozzle
holy crap thats totally do-able!
Yeah I didn’t mean you guys could copy that instructable exactly, I just meant you guys are smart enough to figure something out. Wow I’m considering doing this to my car, I would have to sell my system though, but if I sold my system, I would have money for my project…