first check speakers polarity is correct in both speakers.
test the source at home,, flat without bass treble, notice if treble is controlled,
if yes, yr car speakers didnt match the car environment you may want to hear speakers with less treble..
flat sound is the real sound and a test of sound balance of amp/speakers with environment if the test result isnt good something didnt match, usually speakers make the biggest difference.
talhadon is right you may want to test a cd player and play a proper treble recorded cd not over dosed with highs
the speakers sold are tuned on original recordings not copies.
copies sometimes have more bass or treble or higher output levels