The brake control cable is a must fit if you want to watch DVD's on you head unit. I believe that it should go to ground via the parking brake so that you only watch DVD's when the car is stationary. I think it can be grounded directly or via a switch. hope someone who is more familiar with this issue can give a better answer. It is illegal to have any DVD playing on the front screen(s), while you are driving.
I think the satnav and DVD are totally separate. There should be nothing to stop satnav being used on the head while driving. Not 100% sure about this as haven't got satnav working yet, and if you are using something like waze off the internet, then what would there be to stop you watching U-tube videos also?
1 - the brake control connection from the 3rd part Head Unit (hereafter referred to as '3HU') -as said up thread, it is there to conform with current regulations that forbid the watching of DVD's and similar video output whilst on the move (see this RAC article for an insight to the regulations). By attaching it to the parking brake switch, the 3HU will not show DVD/video on its screen.
If you choose to go down this route, the first challenge is finding the right connection point in the Elgrand to achieve this. If you do find the right connection point then please do publish it as others would be very keen to know where it is. One place to find it is on the rear of the OEM Sat Nav box I believe, and access to the connector is NOT easy. This is why some members suggest fitting a (hidden) switch to the connection (so you could, if challenged, demonstrate to say a law enforcement officer that you cannot watch DVD/video on the move?)
2 - As you have probably found out already, to display the 3HU screen on the OEM front screen, you need to connect the 3HU video output to the rear RCA connectors.
On my Pumpkin 3HU, I have connected to the rear RCA connectors but I am not able to display the Pumpkin's Sat Nav on the OEM front screen and DVD on the 3HU screen: what comes up on the OEM front screen is simply a copy of the Pumpkin screen.
From the postings of others, I suspect that different 3HU may provide the display of Sat Nav on one screen and something different on the other, but I can't confirm this and have no idea which units might do this.