L.S.
She doesn't like the car seat because mommy and daddy aren't paying attention to her. She has realized that if she screams enough she will get you to stop the car and pay attention to her. She is a smart little kid.
First you need to drive around more during the week. and Secondly when you do go out on the weekends -- let her scream. She is 14 months old and she needs to learn to sit in the car seat.