ok think of it this way. Your headunit will have minmum 1 RCA out or up to 3 RCA out. If the Headunit has only one, that RCA output will be fullrange front. That fullrange bandwidth will needed to be divided to the fronts/rears(if necessary)/and subwoofer.

An initial active crossover(powered) will seperate the signal into a highpass signal and into a low pass signal. Typically you'll have 1 RCA input into the crossover and 3 RCA output (front, rear, subwoofer). The front output will go into your amp, and from your amp into your passive crossovers which will further divide the frequencies between your component woofer and component tweeter.

Now if you have a headunit that has 3 pre-outs(front, rear,subwoofer), you can just wire the front(fullrange signal) into your front amp, and use it's internal crossover to attenuate the low frequencies. Same thing with the rear RCA out, the subwoofer(signal already lowpassed typically about 90-100hz) output RCA will go to your subwoofer amp where you can adjust the amp's crossover to further attentuate range.

hope that helped a little.