you might try getting her in partnership with getting into the car seat. Some of the new research out on attachment suggest that focusing on responsivity helps the child experience that the adults understand their needs. When there are conflicting needs between baby and parent, it can be extremely helpful for the us to slow down, get them into partnership as much as possible, saying quietly slowly softly, "sweetie, you don't like the care seat, (slow slow, the infant brain generally needs much more time to process language, and when they are upset they are active in the brain stem which causes even more of a delay in processing). I see... you don't like it (pause). But mama needs you to be safe. "
Let them do as much of it by themselves as they are interested. If you can let her independently climb into the car seat (at appointed no rushing times), ask her to reach for the straps (let her help you). "Can you put it over you head? Peak a boo, there is your head? Where is your hand, can you stretch your hand through the strap?" Also, you can have her climb into it, strap up and then get out again, respond to her communications. "out, you want out? You are crying you want out. Mama knows you want out, but you need to be safe." Lots of slow, simple *repetition* in the talking will help. Perhaps you might try to make some very short trips. "Now we are stopped and you can get out. You want to get out, but mommy needs to stop the car first. Now you can get out." Through responsivity infants understand we are in their court, and we become the container, the holding environment for their fear, pain whatever it is. Usually that is what they need, they don't necessarily need to have all discomfort eliminated, but they need to know that we are with them and understand what they are trying to communicate.
Sorry, no suggestions about the car seats.
Good luck.