It is just possible the wiring is there already, if your car had option 862 (pre-wiring for TV) and then the TV was added, the pre-wiring included the TV tuner to the option 861 rear display.
Otherwise you will have to get the TV harness with option 861 and install the bit from TV tuner to the rear display. There are 3 different versions based on chassis number .. (MY2002 and before (i.e comand 2.5), MY2003 (comand 2.0DX widescreen), MY2004 and onward (comand APS NTG1)
Richard