Hiya! I also have a 2 door Cavalier. We have fit several seats RF in this car! The Marathon fit GREAT RF in the center of the back seat with a lap belt only install as well as fitting in between the 2 front seats GREAT! My DH (this is his car) was able to recline the drivers seat all the way back without hitting the seat at all. We have also fit it behind the drivers seat RF, but it was VERY upright, right on 30 degrees.
However the TrueFit didn't fit quite as well. It seemed to hit one front seat or the other RF in the center. It did fit though, just not as well.
As for the Radian, I can see it being a GREAT fit as well with as slender as it is. I have never tried it though as I have never seen or played with one.
Another GREAT option for a 2 door car would be a Evenflo Triumph Advance. It fits GREAT RF in a 2 door car & still leaves tons of room for the passenger.
I have pictures of my Marathon RF in my DH's 2001 2 door Cavalier as well as pictures of my Evenflo Triumph Advance RF in my 1997 2 door Subaru Impreza!
Marathon in Cavalier, this shows it in between the 2 front seats.
Here it is behind the drivers seat
Here is my son in it at about 20ish months!
Sorry the pics are not better. It is hard to take pics of a car seat RF in a 2 door! The MA is a GREAT fit though in that car! I can try to get pics of the TrueFit if you're interested. I unfortunatly do not have any at the moment.