You guys are correct, rear monitors will mirror DVD, SDCARD, AUX, A/V inputs, but not hdmi. The only mirroring available with hdmi, is to mirror your iPhone, iPod to your head unit.
Very disappointing that you cannot mirror hdmi to rear screens. I originally wanted to mirror youtube kids for the rear screens, and purchased the hdmi to composite adapter. was too much of a hassle to have apple a/v adapter hooked up to phone, then hdmi to composite adapter hooked up to aux 3.5 to composite adapter or A/V input.
If your iPod used the 30 pin plug, there is a composite adapter available, but if you have the lightning plug, it is only digital and therefore video only available with apple A/V Adapter to hdmi.
For the head unit to mirror hdmi, it would have to have an internal digital to analog converter, since hdmi is digital and composite is analog.