Crossover?

I have a '86 stock delco stereo unit that is driving 2 scosche 6x9 factory upgrades (rear), 2 5x7 schoche factory upgrades (front door mounted), and the stock freaky size speakers in the dash. (1986 Buick LeSabre if that helps)
I plan on putting a '93 delco stereo w/cassette and 5-band equalizer in place of the existing unit, and would like to know if it will filter high frequency sounds from the rear channels, and filter bass from the front channels (midrange still played on both front and back channels).
If not, will a crossover do this? Where can I get one?

NO ..... if you go to an active crossover, you will need seperate outboard amps. If you use pasive crossovers, you are gonna waste a good bit of what little power you have.