sorry to hear, that you should have knows that 7 inch is perfect double din screen.
Now you got more problems, and headaches. if you still can can i would exchange it back to 7, personally i am mounting 10.4 monitor in double Din, in my Jetta.
try getting, and external USB keyboard, and maybe cd-rom as well, that would simplify the process, and you would not have to mess with more wires, i am not familiar with forrester's chassis, so i woulnd know, if you could manage mounting laptops, bexing the gloves box on you firewall, it would be nice.
Before you start butchering your car, think if audio too, make sure you can run, audio, and whether you would be satisfied with it.