I think the installation will be fairly simple.
1) Yank a little more on the head unit (the am/fm/dvd/cd player) above the barrel chair. It is held with velcro alone. Just be sure there is a good amount of slack on the wiring that enters that cubbie hole from the cabinet to it's left.
2) Once it starts to come out, continue to feed the wiring from the cabinet. It's real easy from that point.
3) When wiring up the digital tuner, just follow the instructions that came with it. It is extremely simple as you will reuse the wiring related to the TV. There is no need to get behind the TV. All changes in wiring is done by the head unit.
You will have to mount the tuner in plain sight, so it's remote control will work when you point at it. That means the tuner will need to be mounted under the head unit's cabinet like a hanging can opener at home. Generic brackets could be made or found at a hardware store. If you don't know what to ask for, just tell your helpful hardware man what you are trying to accomplish. Bring the tuner with you. Once mounted, you will need to drill a hole in the floor of the cabinet for the tuner wiring to pass through to the head unit and TV. Drill the hole far enough back so the wires enter behind the head unit, not under it.
Imagine the digital tuner mounted to the underside of the cabinet, with the wires going back, then up inside to the head unit.