Well, it's not going to be super fun, but you can make it work :)
I wouldn't go with a Radian; as much as I love them there is NO WAY it would fit rear-facing in your car - the shell is just too tall (making it great for rear-facing to the limits of the seat, which is recommended by the AAP, but bad for your tiny car!). The seat is narrow, but TALL.
I'm not really familiar with the Eclipse myself, so I don't have a ton of advice for you. I would recommend posting over at www.car-seat.org - there are lots of extremely knowledgeable car seat technicians there are I know a couple of them have/had Eclipses. Someone over there is sure to have some good advice.