Here is the post that helped me tap into the Vehicle Speed Signal (VSS) wire behind the instrument cluster. Speaking of instrument cluster, it has to come off, which for me was a pretty scary proposition. In the post below is another link to removing the instrument cluster.
Vehicle Speed Signal - Finally an answer!
I also wired mine to a switch, so it would not limit what I could do while moving. However I have found that I have never had to disable it. Maybe that is just for watching videos while driving. Which I have never tried doing
Good luck....