Overloading constant/switch power leads?

I'm hoping some of the gurus here can help me out. Right now I have the 2004 EX double din HU along with and aftermarket Kenwood satelite tuner running thru an FM modulator. The satelite tuner and modulator are taking power (12V switch and constant) from the radio power leads. I am in the middle of installing a 9" overhead DVD system along with a 6" monitor for the front passenger. Both of these video devices also needs 12V constant and switch power sources. Where should I tap into for these additional devices. I was going to tap into the same radio power wires, but it dawned on me that this probably would overload that circuit. Where should I tap next?