The best and simplest way to upgrade an audio system is to fit a set of front component speakers. This consists of a woofer, tweeter and a separate crossover. The crossover separates the frequencies produced by sound into high frequency and middle frequency. The tweeter produces the higher frequencies and it is best suited either high up on the door or around the dash area. The woofer should be placed lower down in the door area. These are generally fitted in factory locations so nothing is on show.