Most head units have crossovers already built in them. Yes, the deck is the cd playr. To run the speakers you will have to piggy back them off another set of speakers.. just splice them in.
Tweeters can only handle very high frequencies and they would most likely blow if run straight to the HU without a crossover to filter out frequencies below 3000hz or so. Very few high end decks are are able to have a tweeter wired to the deck. Nearly all other decks will put out frequencies to low for a tweeter to handle.