Like many children, your daughter probably doesn't like being restrained for long periods of time. There's also the possibility that she's uncomfortable in her seat, so make sure that it's not too hot or scratchy. Although you probably have tried a variety of diversions already, don't forget to bring some of your daughter's favorite toys, books and activities for each car trip.
Above all, it's imperative that you tell her how important it is that she remains in her seat while you're driving. Mention it to her before each car trip, and at other times of the day. Let her know that she must stay buckled in for her own safety.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.