Thats any easy one, you need an adaptor loom which plugs in to the Video module in the boot.
http://www.toysinyourcar.com/acatalog/rov_av.html
Then you simply run a couple of RCA (Phono) wires through into the cabin (or where ever you decide to site your DVD Player / Freeview receiver)
As regards putting in a Freeview tuner, I tried this with mine, using a battery operated handheld Freeview digibox that I'd already bought from Maplins.
I have to say that digital reception is problematic at best. On the move it is less than useless, but parked up you can sometimes get lucky.
Unlike analogue which will allow a viewable picture even if very poor quality, with digital it's all or nothing depending on signal strength.
Though to be fair I was using the units magnetic antenna stuck to the roof of the car when I did my test, perhaps the vehicle's own rear screen antenna might prove a better solution.