My DS first used one for a 6 hour car journey when he was 2.5. It was a DVD though not a telly. We did not have headphones on them(also have dd who is a year older than ds) We could still have the radio on in the front and it did not cause to much bother.
You attatch the screen to the headrest in front( elastic) then rest the player between the seats link up the wires, plug in to ciggie lighter and away
We just got a cheapy set on offer from argos the night before we went.