This can normally be down to a few things but more specifically its usually down to the quality of the amp and or the rca leads - the feedback is a whine you get from the alternator (used to be a big problem years ago with the poorer quality items available back then but not such a problem now)

What make is the amp ? and are the RCA leads a decent quality as well ?

As mentioned before it can also be something as simple as a bad earth as the whine is sometimes described as an "earth loop"

If all else fails then a noise surpressor is the way forward but again get a decent quality one cos a lot of the cheap ones dont do alot.