Thanks for your input. It turns out that while it was wet down by the amp, no water had gotten inside of it. Instead of having real car speakers in the trunk, I've got one old bigass house speaker box. It was the only speaker hooked up to the amp. On the back of it there are two screws which you connect the speaker wires to. This speaker box is so big, it barely fit in the trunk by millimeters. I had to take out the trunk mat for it to fit. And coincidently, about the same time the water spilt, I had moved the speaker box a little so it wouldn't sound as shitty. I finally figured out that those two metal screws were both pressed against the bare metal of the trunk bottom, shorting it out. And that's why I was seeing sparks on the amp. Case closed.