Buzzing sound through my headphones

Hey guys, got a quick question I have a 98 expedition with an alpine 7892 head unit running a 4 channel alpine amp for the doors and 2 mtx 501d's pushing 2 12 inch typr r's at 2 ohms.  I have all my power wires running down the drivers door and  my speakes wires running down the passenger door, sound good no humming sounds or anything. Now i just installed a drop down montior with the seperate dvd player mounted under the passenger seat, once again i ran  the power wires for that across to the drivers side, and ran the audio and video cable from the passenger side. I turn it on and I hear this durn buzzing sound from the headphones. I am lost