Navigation Unit, Integration, Speed Sensor?

Nav is new to me.  Please help.  Did some research on different units.  I keep going back to the two Pioneer units.  One expensive, and the other way expensive. 

You CAN connect Nav directly into an existing mobile video system with a display capable of video inputs?  i.e. Sony XAV7W?  Can I connect it to the video input on the existing DVD player?

Speed detector?  Necessary for all units?  Is this reading actual speed or calculated speed based on oem tire size etc.?   I ask because I will be plus-2 size on rims with slightly taller tire profile which effects the actual speedo reading which will throw the Nav calculated position indicator off (is this right?).     Or does the unit recalculate position by GPS frequently and make the correction?  

Thanks again Jeff . 

One more...Hope I dont frustrate you on this one...   Have you been on a flight where they show on the movie screen the planes' position on a map relative to the destination?  I would like to have the Nav showing on the rear overhead monitor while still being able to select audio at the head unit.   (Are we there yet? - no, are we there yet? - no, are we there yet? - NO!   type of scenario.) 

I could achieve selectable rear-nav-only by using the aux-in on the existing DVD player directly from the Nav unit?  because....

I have the DVD player directly input the rear Overhead monitor (and output the overhead monitor into the indash monitor controller) - to allow front/rear independent viewing/listening regardless of source selected from head unit. 

This sounds correct to me.  Would like to make sure. 

i have a pioneer navi and i want to put in my truck but i can not find the seed wire for my truck its a lexus lx470 could u email me the diagram pls  thanku