Do you need an amp for aftermarket car speakers ?

You can install them in place of the speakers you have. They will play without an amp, but they can handle lots more power than your stock stereo gives- probably twice as much. For a small car like that, six speakers running  off the stereo should fill it with sound without turning it up too far. They will not extend down to the earth shaking low bass though, amp or not. They are not sub woofers. For music  though, and not war sound effects and thunderstorms, they will do the job. Why didn't you listen to the speakers before you bought them, so you'd know how they sound? Those speakers can handle about 60 RMS watts each. If  that doesn't suit you, shop for a sub and a 500 rms watt or more amp.