I didn't see the previous post (I don't think), but I voted "over 2 years" b/c dd is 2.5 and we are nowhere near turning the Marathon FF. She is petite and only weighs 24 lbs. She will probably still be RF at 3 yo!
Oh dear, just read on past.
It's hard when a babe hates the carseat. I have thoughts but have to go, back in a while.
Edit (just came back):
I don't know how old your baby is, but things that have helped in the past - I was just talking to someone today about how much Nora used to hate being in the carseat. FWIW, I'm not sure FF would help, if you are considering that. (But if your child is 1 year + 20 lbs. and NOTHING else works, you could try it - you obviously know it's not as safe all things being equal, but I know that a non-screaming child could possibly make the car ride safer overall than a screaming child).
--baby is probably too old for this, but REALLY loud white noise (static) on the radio
--a big bucket of interesting toys and books in the front seat which you toss back as needed. We used to have, like, 20 toys at a time ready to go.
We'd use 'em all, too, sometimes in a 20 minute drive!
--let baby hold something normally forbidden, like the cell phone (turn off first?)
--obviously only when possible, one parent drives & one parent sits in the back.
--singing really really loud / CDs. try all different kinds of music. try talk radio. try NPR. something might catch his attention.
--mirror with blinky lights and music that you control by remote control, made by Fisher Price. If you want I can find a link to it. I think they sell it on Amazon.
Best of luck.
Another edit: if this is a "new" problem, it will most likely be relatively short-lived. I hope so!