What monitor are you referring to.Originally Posted by the_realscott
Basically, ground is ground, so you can probably conect them both together, adn ground them adequately. Never really heard of "accessory earth", though they could just be refering to the ground that is for the accessory part of the circuit.
As for the ignition, generally the accessory wires of a device are low amperage, adn therefore, splitting them should not be a problem. Same with ground, but if your device requires a lot of power/amperage, it is better to have adequate ground. The screen probably doesn't require much power (probably an amp or so) so you should be fine splitting the car radios ground.