When your baby is sweating in the car seat it means that the baby is too hot and needs to be kept cool.
Before placing your baby in the car seat make sure the car has cooled down enough if you have air conditioning.
So allow the air conditioning to run for a few minutes before putting the baby into the car seat.
Also use those baby sunshades on the side windows or have tinted windows to keep the sun from hitting the baby in the car seat which can make them sweat.
When putting the baby in the car seat dress the lightly and if it's hot out you could even just put the baby in the car seat with only a diaper and shirt or just a diaper.
This is what I did in the summertime when my cars air conditioner was broken.
My newborn baby and my 2 year old boy would just wear the diaper while in the car seat.
The newborn wore just the diaper and my 2 year old boy had a shirt on as well as socks and shoes but wore just the diaper which kept him cool enough.
He wore a shirt because the straps on the car seat would bother him and leave a rash on his shoulders.
You can also buy those 12 volt portable car fans that you can hang in the car somewhere and allow it to blow on the baby to keep them cool.
Or a USB or battery fan can also work to cool down the baby or toddler in their car seat.