I have a friend a hurt his back by take the car seat in and out of the back of his bimmer coupe. As soon as the lease was over, he got a 4dr right da way.
And I had a baby seat in the back of my G sedan not too long ago (we bought another family car since then). Its not the most spacious backseat, but you will appreciate the extra doors. and they almost open to 90 degree!
The baby is not too heavy, but the car seat along was +20lb!