I had a 2 door Yaris up until right before Christmas, and I agree with PP that while it wouldn't be impossible to get a carseat in the backseat, it would be really, really hard. I really loved my Yaris too, and I didn't want to part with it, but I honestly didn't think I'd be able to get a carseat in/out of it without physically crawling (at least partially) into the backseat. In the end, I traded it in for a crossover.
if you really want to know before getting rid of your Yaris, maybe you can borrow a carseat from a friend or family member and try it out. And don't forget to add a few extra pounds to factor in baby!