Which mount are you wanting to use? There is two. One with secures ONLY the top of the vest and requires the vehicle belt to pass through loops in the front of the vest, over the child's hips, or one that secures both the top and the bottom sides of the vest in such a way that a child cannot undo themselves like they can with the seat belt in front method.
We use both the E-Z-On 103Z and 102 Vests in our family car. As
safeinthecar has already said, you will need to decide which mounting system to use with the vest.
When making this decision it is worth noting that the first option – the
Tether Mount (which attaches to the vest only at the shoulders) – requires only one anchor to be installed behind the vehicle seat. The second option – the
Floor Mount – requires two tether anchors to be installed behind the vehicle seat. This can best be seen by viewing the instructions for each system:
•
E-Z-On Products Tether Mount Instructions (uses one anchor)
•
E-Z-On Products Floor Mount Instructions (uses two anchors)
These requirements could affect the installation of the tether anchors. For example, it might be possible to install one anchor behind the seat, but perhaps not two anchors (or vice-versa).
In short, the choice of mounting system depends on the vehicle design as well as the needs of the passenger. It is worth considering this when selecting the mounting system you wish to use.
I hope this helps.