Do you think it's possible that you are doing too much? I ask because my daughter learns quickly what will get her attention. So I just let things go and don't fall all over her when she's having a fit that's not life threatening. If I give in, she knows she will throw a fit and get what she wants the next time. This is why I don't negotiate. If she cries, she cries. Obviously I try to make her as comfortable as possible. But if I've done everything and she's just throwing a fit.... Well, she can throw a fit. She has to just get used to the fact that she's going in her seat rear facing and that's that. In her life, she'll be in these situations time and time again. Sometimes she can get what she wants, and sometimes she can't. She'll have to deal.
Also, my coworker's daughter threw fits in her car seat. So my coworker didn't use a car seat AT ALL until her daughter was more than two (I was not aware of this)! She carried her or put her in a regular seat belt just to avoid the fits.
My coworker one day was just too scared to not put her in her seat. So she put her in it, her daughter got so upset she puked, and then she's been in her seat ever since. I mean, I feel for the kid for being that upset. But she learned that it was just the way things were going to be.
I hate to sound mean or uncaring, but hopefully you get where I'm going with this. As parents we have to keep our children safe, and sometimes children aren't going to like it. And my daughter is well loved, loving, well adjusted, and happy.