Rte66 is right, you have to decide how much you want to spend. Maybe do a little bit at a time to see if the improvements are enough for you, first start with replacing the speakers.
I went ahead and did a replacement of speakers, added an amp, and added the Mopar sub, along with sound deadening, and the stereo sounds excellent now. I even removed the little dash tweeters, they didn't seem to be doing much anyway. Adding the sub helped, but not until I added the sound deadening to eliminate road noise and vibrations could I really crank up the sound. The amp helped to get the most out of the new speakers.
I'm sure there are a bunch of stereo people here who can give you better advice than me, but I would say at least start with some speakers.
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