You might look up the safety tests they did on tethers and such a while back. I think they all failed during sudden stops or simulated crashes. I just put Tibbe in his buckled in airline carrier which also protects him from the airbag as we don't have a backseat for him to ride in. I don't know if a harness that a seatbelt fits through would be more reliable or not but it might since there might be less torque - IDK.
Pet car safety restraints found to be less-than-effective in recent study