We bought a new car (Subaru Outback) to solve this problem. :) It has more space between the front and back seats so my DH is able to drive it pretty comfortably with the infant seat behind him and convertible on the passenger side.
Maybe see if there's a different convertible seat you could buy that would allow you to put the driver's seat back further? Some are smaller than others and might not block the front seat as much depending on their shape.