I am thinking to install HID on 2009 2.4l Sonata, but I am not sure where to fit the ballasts. The space is really crowded behind the headlights. I know that some users did the conversion already; could you post a picture of your ballasts ?
get the slim ballasts (most of the HID kits today usually come with them). It will fit in the gap behind the front headlights. Use some tie-wraps to secure the wires so the ballast doesn't randomly move around when you are driving.
You will also need to drill a hole in the cover / cap that leads into the main headlight (H7) housing, so that wires for the HID bulb can go through.