S.C.
unfortunately,she's just gonna have to deal with it.as you said,safety is the issue here and that's not something you can compromise.however,i did see something that she may get a kick out of(or not!).it's a little baby doll car seat that i believe you can strap into the car.she may get a kick out of having her baby ride in a car seat with her.then again,she may not.
here's the link to one at target if you're interested.
http://www.target.com/gp/detail.html/sr=1-1/qid=116179277...
it's not too expensive and can strap into the car with her and can be used as a little backpack.just something that may make riding in the back a little more interesting. also don't allow her to play in the vehicle at all.be consistent about what you do and sooner or later she will accept it and won't even think about it.hope this helps! ~S.~