A deck install is literally the least difficult thing to do. Especially in these subarus.
Take the clarion harness you have, and literally color match what goes to the subaru adapter harness you purchased.
You will have a few extras on the clarion unit (brown, light green, purple/white)
Brown is mute. Do not hook this wire up.
Purple/white (I believe, it may be just purple) is your speed sense wire. Unless you want your deck to show your speed, which honestly who cares, do not hook it up to anything, just cut it down)
Light green. This is your parking brake wire. You can do one of two things with this. Hook it up to your e-brake wire. This will mean you have to pull up your ebrake to enable watching video, or adjusting your navigation while driving.
Other option is to hook it up to a switch for continuous video, or use a TR7 module for continuous video.
If you were local to me I'd take the 10 minutes and do it for yah.
