RCA cords in amp create loud whine

I have just purchased a DVD player and mounted it below my factory stereo system. I also purchased an amp. The amp is powered directly from battery and grounded to driver's seat bolt. The stereo's speaker wires are wired into the high input section of the amp. The DVD player has RCA cords, and those are connected through the amps low input section. Now there is a horrible whine that increases in pitch with RPM's when the engine is running. Does anyone know how to stop this? The whine only occurs when RCA cords are plugged into the amp (the stereo works well with no whine). It seems that the "hot" wire for either the stereo or the DVD player causes the whine. It doesn't matter if the DVD player is not wired into anything, as long as the unit above it is wired, I still get the whine (and visa versa, if only the DVD unit is powered by a hot wire, then I also get the whine). The two units (DVD player and stereo) are not wired in any way together. I was pulling power from my cigarette lighter to power the DVD player. But like I was saying, the whine still occurs if the DVD is disconnected from cigarette lighter and has no power to it as long as the stereo above it is plugged in to the factory connection, I still get the whine through the unwired unit below! Any ideas?

you can't do what you are trying to do, you can EITHER connect up the cars' stereo to the high levels OR the DVD to the low levels. Since the audio from the two sources has two diffrent gounds, you are getting a ground loop thru the amp.

Thanks PaulD. That was it! As you might have guessed, I am totally new to wiring audio. There is still a tiny whine when I disconnect the other, but that is no doubt because in my mind, I thought more grounding was better and grounded to about everything metal I could find! I'm working on making a switch so that I can manually break the connection of either the stereo or DVD player without having to plug and unplug units. I may also purchase a ground loop breaker just to try to get rid of the last of the whine. Thanks for taking the time to write. Now I can watch movies as I camp in my car. Got S-video, VGA, and RCA video outputs so I'm set.

this is an interesting thought, but a preamp device might be more useful. Be careful of continously disconnecting the outupts of the cars stereo and the DVD player while they are on.