Your first mistake was going to Best Buy. Take it to a reputable car audio place. If it originally had an aftermarket amp in it and now instead of the subs running off the aftermarket amp they are running off the stock amp or the head unit this is something you never ever want to do with aftermarket subwoofers as you will blow up your stock amp and or head unit.
Also a picture of the wiring they did would help. And the model number of the radio its usually printed right on the front of it.