Let me first state that I have a 13, 10, and 7 yr old and all have rode in my
stang at one time or the other in car seats. I found a good video for you to watch at
Installing a Car Seat#
You will need a locking clip as it shows in the video. These can be purchased from wal-mart in the car stuff or the baby department. First route the seatbelt thru the carseat per manufacturers instructions, then once you have the belt tight, hold the seat belt so not to lose where the two belts are together at the clip. Then install the clip onto the two belts of the seat belt. I usually tighten the belt just a little before putting on the clip. This way it is good and tight in the car. Make sure you put down a towl between the seat and the car seat for protection. Ok, now that you have the clip on the seat belts and the seat belts are routed thru the car seat, position yourself to appy pressure to the carseat thru hand or knee or whole body so that you can clip the seat belt into the receptacle. You want the seat to be very tight into your car's seat. In the
stang I neve hooked up a tether, so can't help you there. It would be different between a hatch or a notch though.
Do a google search and you can find lots of info how to put a carseat in.
This should get you on the right track. Let me know how it turns out.