DVD player won't turn on with ignition.

O.K. I've had a couple of stand  alone dvd players that have all done the same thing. The problem is as follows, If you turn on the ignition the power light goes on but the unit does not actually power on until you hit the power button. What I'm looking to accomplish is to have the dvd player and the headrest tvs to fully turn on with just the ignition. I wired everthing right and have the constant power wires and ignition power wires going to the right sources. It seems like that is just how the system is made. I've had other units in the past such as Alpine and Pioneer and they act just like a regular aftermarket radio, you turn on the ignition and they fully turn on. Well if anybody has any advice I appreciate it in advance. Thanks 

Mike

If you find out lemme know!