I have had two babies who hated the car seat too. Both were heaps happier when they were eventually forward facing (it is recommended to wait until they are 12 months to turn them around though).

Are you still using the infant insert in your car seat? If so, try taking it out and seeing if that makes a difference. My friend's bub screamed when in the car until the paediatric chiropractor (of all people LOL) said that some bubs hate being so squished in the padded insert. She took the insert out and had an instantly happy baby! She hasn't had a problem since .