I'll let someone else answer most of your questions, because my brain is off somewhere else.

If you have component speakers, which it sounds like you do, the component crossover takes the amplified signal (from the amp obviously) and splits it into the right frequency range for each speaker. So if you have a 3-way component set that has midbass, midrange, and tweeters, the component crossover (one for left, one for right) will have one input that comes from the amp, and three outputs (a midbass, midrange, and a tweeter). The crossover can go just about anywhere you want it to, but it's usually easiest to mount it in close proximity to where the speakers will be mounted. So if you have all of the speakers in the doors, the crossover can be mounted in the door so you can run one speaker wire (from the amp) into the door from the chassis. It's really just a matter of mounting convenience.

Is that what you're asking?