Well after checking the ground situation, there are some things to try. First, find where the noise is actually coming from. Disconnect the audio RCA cables from the PAC unit. Do you still have noise? If yes, then the PAC piece is bad. If no then reconnect the RCAs but disconnect them from the DVD. If you have noise now, scrap those cables and get a good set (Stinger, etc.). If no noise still, plug in the DVD. If then you still have noise then:
1- You have a bad DVD player (noise is somewhat typical for an Audiovox DVD, but not hard to get rid of)
2- Ground your DVD to the radio.
3- Add a GOOD RCA noise filter (PAC SNI-1). Did they try a good one?
4- Get power for the DVD at the radio harness. In my FX i got power for my Nav-TV at the lighter socket in the console at first, but it was a little noisy.
5- Did the installer cut the power line filter off of the DVD's power harness? Sometimes i cut them off, but need to put them back in for noisy vehicles. You may need to add one.
Engine noise is sometimes a pain to get rid of. If none of this works, you may want to try another brand DVD player. I'm assuming they put in a DIN size unit? (AVD300, MVDVD4 or similar) The Audiovox is cheap, and usually good. But sometimes you get what you pay for. Many aftermarket DVD units are Din size and should fit in the same hole with no modification. Wiring is virtually the same, but you may need to splice on the new power plug. Hope this helps.