I have a 2008 Tarago with the built-in sat nav & overhead DVD. I'll will try to answer your questions as best I can... "First, the book says the unit remains on but shuts off the screen when you close it, however it also says you can turn the unit on and off with the remote. Could it just be that the DVD player needs the remote to turn it on or should it power up when the car turns on or if I insert a DVD? It does not appear to have any power at all." This is correct. The DVD player is always on as long as there's a disc inside. There is no power button. All I do is turn on the engine & push a disc into the drive & it just works. No need to do anything else. "Second, the previous owner has obviously upgraded the sat nav unit as it's not the stock one from the original owners manual. I do however have an owners manual for it. It appears he upgraded a lot of options when he bought this car. Is it possible that they didn't hook up the DVD player properly when they installed it? The disc/aux button doesn't seem to see the DVD player and neither does the mode button." The disc/aux button doesn't register the DVD player until there is a playable disc in the drive. If no disc is inserted, pressing the disc/aux button just cycles between the CD player and the AUX channel.