Ive been researching this for quite some time. You could get away with a custom res in the front lower bumper area, or the most sensible place, is the rear trunk, where i plan to mount mine. I would like to have at least a 1 gallon tank.
If i can find a decent location for the pump, than its going into the engine bay. Im not too jazzed about running a large power cable to the rear of the car if i dont have to. Wouldnt be a big deal if i had amps back there, but i dont.Not much room in the engine bay, so its going to take a little bit of work to plum it in.
Others have used the windshield washer tank for their installs too.
Im planing on installing mine were the car jack is in the trunk...thats were the tank will be....and the pump as well just gonna have to run the lines all through the inside of the car but it will be a clean and stealthy install....