Right rear of the car near the fuel tank. It's black and about shoebox size with some valves and hoses going to it. one hose goes to the fuel tank.
I doubt water is getting in from the outside, unless it's cracked or there is a leaky hose or something. It could be from moisture in the fuel system though.
We have seen the vaccuum switching valve on various models cause a check engine light...yours appears to be part of the canister. I guess the valve will stick sometimes causing the light.