I installed OEM rear speakers over the summer and they made an improvement for me. If you want to upgrade the speakers any 3.5" speaker should fit, and you might be able to fit dedicated tweeters in there, I know a few people have.
Have you tried adjusting the balance? If you can't hear anything from them, turn up the sound from them, I had to mess around with mine, until they filled in the cabin nicely.
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1999 Ocean Blue Metallic Boxster - blueboxster.com
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