Sub has no highs but play lows nice?

So I had my 500 watt alpine amp with Rockford punch 12" in my hyndai with a head unit and it hit the highs and lows perfect. Now I transferred the same sub and amp to my Ford Taurus only using a converter so I can play it off my stock radio. The lows hit so nicely but there's like no highs at all.what's up with that?

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