No help here -- but you're not alone. Guinevere STRAINS against the straps now. She wants to sit up so badly -- it's not just the car seat. She's started straining in the swing too. And of course, we get her out of the swing when she does this, but she's miserable in the car seat now and there have been times when I've made DH pull over and stop the car so I can get her out, she's so miserable.
The only thing I have been able to do is sit back there with her and let DH drive, so I can try to distract her. But that doesn't always work.
Surely it's a phase? Winnie can't sit up by herself yet, but she wants to be sitting upright as much as possible, and the car seat taunts her greatly.