My DVD Override kit will help in that it will allow you to display the screen (via the Aux socket) when driving. However, when you select Aux it will switch the audio to Aux too so you won't be able to listen to the radio, CD, etc.
If you had a non-Premium Connect car then my other kit would be perfect, but that doesn't apply to you unfortunately.
Something I could suggest is for a different option where I use the reverse camera signal to display a different input. It's turned on and off with a switch and allows you to use the stereo for other audio when in use. The snag is that by default the reverse guide lines are displayed over the image, but you can turn them off in the Connect Premium menus (but then you don't have them for reversing...)
Does the Carbonsignal device only output Composite (CVBS) or there any other outputs (HDMI for example)?