This typeof question usually come sup when discussing the issue of using a GPS in tracking the whereabouts of a wayward spouse by another spouse. You know: having him followed to gain evidence of adultery.  In those instances it has been held that it is legal and not an invasion of privacy as long as the tracking was done on public streets and as long as  the spouse has some sort of financial or legal connection to the vehicle.  There of course also must not be a law making it illegal.  The factors I mentioned before are from a case in a state that has no laws on GPS tracking.  Now, your case is a but different.  It involves monitoring by someone other than a spouse.  I think that you should speak with an attorney on the matter.  I think here it could indeed be an invasion of privacy.  Good luck.