She must use it. Make some colorful toys available, make sure the sun is not shining directly into her eyes, make sure it fits her appropriately and that she's not too cold or too hot. At that point, just let her cry and try to avoid long trips without frequent stops. She will get used to even unpleasant experiences if there is no choice.
Minimize Trips! Sadly, babies don't understand that the car seat is for their protection. His/her crying will not be harmful to the baby. If the baby must be transported, make sure all their needs are met prior to departure and ignore the crying. Sometimes an older infant or toddler will be calmed by music. Take breaks on long trips. Never drive anywhere with the baby unsecured in the car seat.
Accept the Obvious. As a cpst tech for a decade i find many of these parents are ambivalent about the seat & extended family may be no help. Having cared for infants ejected thru windshields, quadraplegic from improper seat use etc. I tend to be blunt. Until you can embrace the need for the seat you have a problem. Your ambivalence can feeds into the babies reaction. Remember this is a way to see baby grow up.