My son cried in the car until he was a year old and forward facing. Things that sometimes calmed him when he was very small: very loud static or music on the radio, a bottle of pumped milk.
I'm going take a different approach, and it's coming late in the discussion. Crying it out is a terrible thing to do to a baby. They are in great need of comfort and asking for it, and intentionally refusing to give this to them is, in my opinion, criminal. That said, babies will cry, because they are really traumatized by being in the world, they are so brand-new, and that is how they communicate. I honestly don't believe that our jobs as parents are to be perfect at every moment, as a matter of fact I see this search for parental perfection as an act of self-aggression. If you can't be preventing crying at every opportunity, you have not failed. So you do the best you can at all times, and you be gentle with yourself when you're not 100%, and maybe you'll reach 80% on a good day, but all of that is better than parenting with the idea that your child should bend to YOUR will, should be convenient and that there are no consequences to your parenting choices.
The car seat is one of those times when it is simply impossible to meet that baby's needs. It's a necessary evil, and it's one that has brought me to tears many many times. I know it's not optimal but I also know that there is something to be said about our voices, even sometimes our touch if you're flexible enough, or our songs, that do help a little and do tell our kids that we are there even if we can't be holding them at that moment.
A happy baby is one that has a happy family. A happy family is one where the needs of all members are met as much as possible. This means that the older sib may sacrifice some things because of the baby's needs, but it also means that the baby might not get the same level of interventions that a person can give to a single child. I know that the transition to being a big brother has been really difficult for my son, and if having camp twice a week would offer some salve for his sweet wounded soul, I would do it. Even if it meant some crying in the car for the baby. Because that baby is more resilient than people realize. The kids who aren't as resilient tend to be older kids. Look at babies coming out of horrible conditions in Russian orphanages - the ones adopted younger can grow up normally, the ones adopted older, let's say around two, end up with higher rates of RAD. So yes, cortisol will be released in the car, and the baby will hear mama's voice but not feel mama's touch. The baby will be really happy to be back in mama's arms, and the baby will survive, because that's what babies do. They're resilient and flexible and adaptable. The most important thing is that the baby will form a secure attachment to its parents, and a car ride twice a week is not going to change any of that. And trust me, I have gone through hell and back not crying it out with my kids, and I am deeply opposed to it. I still don't see the necessary evil of the car seat as crying it out.
So, OP, I'm really glad to hear things are going better. Ease up on the guilt, mama. You're doing the best you can and it's a hard thing to go from one to two kids. Your baby will be fine and already feels so loved. I just think about the teenagers I saw once on the bus with a baby in a car seat. The baby was never touched or held, no one looked at the baby or talked to the baby, it just sat there, not talking or crying. There were four girls with the baby, heading to a high school basketball game, and there was no way I could tell which one of them was the mother. That's pretty uncool. A baby who hates the car seat and a mama who is so upset about it that she's crying - that's an empathetic and caring parent. Word.
Queer Parenting since 2007
I had a car crier, and I know, it's awful, but you can release this particular worry.
grateful mother to DD, 1/04, and DS, 2/08