I put my crossovers in the trunk, makes it easier to get to if I want to attenuate my tweets or whatever else the XO may have switches for. Also because I plan on having a show trunk, and want to show off the 4 xo's it takes to power the rainbows. The downside is that I have to run a lot more wire.
I would use 16 gauge speaker wire, do not, reuse the stock wires, they are junk. And as mentioned, 4 gauge will work for power/ground, although if you ever decided to upgrade, you can use 0 gauge as well.
www.knukonceptz.com has the best wiring for the best price. I would avoid putting the crossovers in the doors, you don't want condensation to get on them. Inside the cabin is fine of course.
I saw an install a few weeks ago that put the crossovers into the doors, right where our curtosey lights are. It was neat looking, but more for the skilled installer.