you could use rca Y-splitters to split it up to the different speakers, and use the crossovers on the amps, otherwise, you could get a active crossover unit-- like the one made by clarion(currently my favorite)-- it can take 1 stereo input and divide it up to 3 pairs of outputs(i have one from my old setup that i am using for a auxilary sound system input in case my computer stops working)
or you could just get a cheap sub crossover-- most of them are under $30.
it really depends on how much room you have, your budget, and what you expect out of your system.
A friend of mine is thinking about doing something with this in his lexus:
Now, if I make my display case out of 1/2" plexi... mineral oil + the bubbles would be pretty cool for show... thoughts?