My little bub goes to sleep in the car so these things are off the top of my head.
Have you made sure she is comfortable eg the seat fits not too tight or loose and she isnt too hot or cold.
Put some bright toys in her sight so she has something to look at
Put some baby relaxing music on that she likes outside of the car.
Make sure she is fed and changed just before getting in. (my baby always seems to go once he is in otherwise and then gets cranky)
Try a densensitisation type program where you work up to what you want. The point being for her to feel a bit more comfortable in the car. Eg. sit in the car with her on your lap and have some cuddles. Put on the relaxing music. Then work up (over a few days/weeks) to put her in her seat for a few minutes with some favourite toys or lots of soothing attention and then to going for a short drive. You could also at the beginning have someone sit in the back with her so she is within easy reach of comforting for them while she gets used to car trips.
HTH. Good luck!