There are a lot of things happening with GPS Tracking. In a nutshell, in Texas as a private investigator, you can put a GPS Tracker on a vehicle with the owner's permission. Police need a court order or owner's permission. As far as State law goes and case law goes - what I know of as of today, there have been no clear-cut cases determining what the term owner means. Texas is a community property state - so even in cases where both the husband and wife are not on the title - they are still both owners of the vehicle. This is obviously a factor in infidelity or divorce cases. This is important because in Texas the "right to privacy" is a pretty big deal.
Things like "the right of privacy constitutes a legal injury for which a remedy will be granted." and anything that goes beyond the "expectation of privacy" can have dire consequences.
Chances are, as long as you have consent form an owner - you are likely to be OK from a criminal or Penal Code (16.06) violation standpoint.
However, now the Supreme Court of the United States is getting into this. We do not want to be violating Federal Law. They decided to look into a criminal investigation by the Police, but we may need to look into it more carefully for private investigations concerns. The US Court of appeals in DC put it this way: "A reasonable person does not expect anyone to monitor and retain a record of every time he drives his car, including his origin, route, destination, and each place he stops and how long he stays there; rather, he expects each of those movements to remain 'disconnected and anonymous," according to circuit judge Douglas Ginsburg.
The case has not been heard yet, and it is about the warrant-less use of GPS trackers by police - but keep an eye on the case as it can have implications for the private eye.
Cnet has the story here.
Whatever you do - Stay Safe!