first of all way to go mom for not resorting to the cry it out method!!! im glad to know there are other moms like me who are sensitive to the needs and feelings of their children! :D awesome work! and keep it up, it seems so hard sometimes when you are so tired and just want baby to stop crying...
hm. this is a tough one. as long as you also dont lose your resolve and loosen the seat belt, this is dangerous and could cause serious injury or death if there were a car accident.
one of the things that they did in the hospitol when we first put our son in a car seat was to grab it at the head and foot and swing back and forth with the seat at your waist. many many many times when our son was young, one of us was in charge of standing and swaying the car seat while the other got the diaper bag and whatever else. once in the car and moving, our son usually was fine, but that initial getting him in the seat was the hard part.
with this subject it is important to show no other option. children must be securly fastened into a car seat and seat belt, no exceptions. if you do it once, they will see the inconsistency and expect to be able to get that again...
this is of course when they are older.
i dont really know what to do specifically about the crying when in the car seat, but give the swaying a try.
it was the only thing that worked for us.