in the boot on the left side when you remove the panel you will see the media rack, they are not all mounted the same but you would normally see the radio on the outside just below the nav dvd, the tv is mounted behind it, its also where you will see the dab unit etc.
you didnt say if you had the analogue tv already - if you have you can swap it for the hybrid unit, pricey but a straight swap, but you will need to unbolt the rack to get the old one out and new one in
if you go aftermarket digibox as i did before i put the oem hybrid in, it leaves you with a couple of issues, 1) you will need to add at least one if not 2 powered antennas to the back screen to get reception, (these can be on the inside) you cant use the original antennas inbuilt to the car. 2) you will also have to get one with a remote eye so that you can control it, 3) if you dont get a half decent one it wont track the signal which means you will be forever tuning it in.
connecting it is easy though, you have 2 inputs available on the tv AV1 and AV2, you may need to add these to the red connector block if you dont have them, they dont come as standard. so using these you can simply output the tv digibox to AV1 with standard connectors and you will have sound and audio.
there is not a great deal of room on the rack so also make sure you get one that fits. i put in this half DIN DVD SD/USB player and it just about fits
http://www.ebay.co.uk/itm/D0009-Eono...item1c372341fe
also you will need power, which i got from the fuse board near the rack, to make sure the power can be switched of you will need to add a switch from there in to the cabin to make sure it doesnt run your battery down.
reception may also be a problem with an aftermarket, it did work but nothing like the oem kit.
if you dont have the tv, you would need to do something entirely different to get the video and sound to the car, there are a few aftermarket products that do this OR you may need to add a couple of modules you wont have if no tv is present
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A8 4.2TDI - Ice Silver, Comfort heated/cooled/massage Seats, Alcantara, Extended Leather pack, Ambient light pack, Solar roof, Night Vision, Lane change assist, Audi presense, Audi BrakeGuard, ACC Cruise, Blinds, Quad Air, AMI, Online services, Parking with rear Camera, Aluminium brushed inlays, Adaptive Zenons, High beam assist, Bose Sound, and more
Gone but not forgotten: A8 D3 2005- AMI HDD, Audible Parking, TV, NAV, Bluetooth, Hybrid TV, Aftermarket headrest monitors, Rosstech VCDS / Vagcom