Not going to get the sound you want without at least an aftermarket headunit and preferably an aftermarket amp. The stock unit is bleh as was already said and the stock amps are only barely enough to make the stock speakers sound decent.
And to get the best sound you will need components in the front and back. Make sure to aim the tweeter right around where the heads of yourself and the passengers will be respectively for each tweeter/seat.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.