The first thing to get the radio out from the car is to pull the air vents and the climate control apart. You can do it with your own hands but it’ll be better if you use some kind of flat trowel. The radio is only fitted to the chassis with 4 bolts and after unscrewing them you must disconnect the ISO connectors plugged at the back side of the unit. Depending on which system you had the connectors panel may change.
The front face is a plastic piece fitted by 4 torx screws and 2 red connectors that interface with the main board inside the radio chassis. Once you disconnect them, you will see 3 different PCBs, one bigger for the DIC panel and 2 side boards for the control knobs. They are fitted with torx screws to a plastic mount as well. When all the parts are pulled apart, you can realize the actual size of the plastic you can cut off to room the screen bezel. Though I wanted to keep the original function keys, the guys I paid to do the job misunderstood me so they cut them off too so I had to change my plans a little. Finally I kept the buttons at the screen bezel so I could change the video source to get the images from the rear camera or turn it off manually.
This is a guide to integrate the screen, but be advised that the chassis from the factory radio is big enough to size it is easier to locate the boards inside. http://www.mechatroniks.com/wrx/lili629.html
Since I couldn’t get a spare radio in the same colour mine was so I had to perform some extra jobs like sanding and painting, so if you intend to do it the way I did do not make the same mistake. It looks great at the end, but it makes you waste a whole week.