P, my middle child has been diagnosed with what would have been PDD-NOS years ago, now the diagnosis is simply ASD (Autism Spectrum Disorder) mild (so she's very-high functioning autistic). With this diagnosis comes SPD/SID (Sensory Processing/Integration Disorder). How our journey towards this diagnosis began, starts with infancy, in particular, her constant screaming over seemingly nothing and our inability to soothe her.
At first she seemed like any normal higher-needs baby, she just liked to be held and comforted, but it turned into being overstimulate by everything and crying constantly. In particular, the car seat made her totally inconsolable.
She had a Graco SafeSeat1 (similar to the current Graco Snugride 30 and 35, it had a 30lb, 32inch limit on it so I thought I would surely use it until she was a year old--boy was I wrong). Even with the infant insert in it she cried constantly.
So...what we tried:
We tried with the insert, without the insert, with the strap covers, without the strap covers (nothing aftermarket allowed, only those that came with her seat at the time of purchase). It made no difference with P at all. It did however make our next child, B, much happier to be in the seat without the infant insert and strap covers (check your manual to see if any of the inserts or covers are required).
We also tried having the air cooler and warmer. Still she cried, though not quite as hard when she was cooler...so we always made sure not to overdress her and keep the car fairly cool, This held especially true for days when she was already upset as she would work up a sweat as soon as the crying and screaming fits started.
The doctor and I tried medicine for colic/gas and reflux, but they did little to help her, though it does seem to help some kids, particularly since infant seats have a sort of "C" shape to them that makes kids with reflux more uncomfortable, a more upright and "L" shaped convertible helps them normally.
We tried music, white noise (there are phone apps for this), rolling the windows down, me singing, toys that had bells inside a stuffed animal or doll, loveys, etc., all to no avail...still she cried every time she was in the car seat, no matter how tired or close to sleep she was, how awake and happy she was, still she cried.
We tried me sitting in the 3rd row of our van when possible, she already had an 8 year old sister sitting next to her that she could see and who tried desperately to console her.
Next we went with static cling window shades to block the sun, that did help some in the spring and summer, but really, still she cried.
At almost 7 months we decided to try a convertible seat. I don't know if it had to do with being up higher and seeing out the window more, the age, the more upright seating/"L" position or the fuzzy soft fabric, but we became a better rider at this point. Still, 70% of the time in the seat we were in hysterics...but hey, that 30% of the time reprieve from crying was very welcomed. Somehow over the next 6 months she slowly cried less and less.
Here's the deal though...I don't think people realize that some kids, regardless of what you do, are going to cry no matter what in the car seat. All you can do is your best: try anything and everything that's safe.
At the time we were dealing with this the Noggle (review by CSFTL.org) hadn't been released yet. I think it could have been a very valuable tool for redirecting air around her. I know that if I didn't have a van with rear air/heat I would buy one now to help cool the back of a car or SUV during our hot and humid summers.
Try it all, white noise, music, rolling the windows down, changing the temps, using a noggle, sshshing them, sitting in the back with them, toys that make noise or light up but are still soft and safe for the car, a mirror so they can see you (as long as it's well anchored and couldn't be a hard projectile to hurt your child in an accident--if you threw it at them would it hurt them?), changing how you dress them, a different seat, installing your seat more upright if it allows, or more reclined if it's very upright already, make sure your straps aren't too tight, but are just tight enough to pass the pinch test, swaddle them in their seat, and just know you aren't alone. Some kids will outgrow it in a few weeks, some a few months, some longer, but as long as they are strapped in correctly and you have places to go, you just go...do plan to make more frequent stops when possible, try to get them sleepy in hopes they'll fall asleep on your journey, and make sure they aren't hungry when you start. There are some essential oils (lavender and sweet orange or roman chammomile) that might be able to help calm your child by placing a drop on a bib or using an in-car diffuser...be careful with essential oils though, make sure what you choose is appropriate for young children.
As for the SPD/ASD, The constant screaming for us, since most things didn't help was mostly sensory related. She didn't like the feel of certain fabrics and would cry incessantly when she wore things that weren't cotton knit. She didn't like the feel of the motion of the car, she didn't like the snugness of the harness (still doesn't at age 5.5). Didn't like the bright sun, road noise, or anything near her head. She doesn't feel temperatures the same way (or pain) that normal children do. So I think a lot of her stopping the crying was learning how to navigate her world that was so crazy overstimulating to her.