Word of advice from a professional... unless you have a very good understanding of electronics and more importantly taking interior panels apart, don't try it yourself. Minor mistakes can cost LOTS of money.And don't trust somebody who has done a few installs and acts like they know how to install everything... it's your car, make sure it is done right... otherwise you'll be doing it all over again later and most professionals will end up charging you more for having to undo everything that is wrong. Although I may not personally agree with some shops methods, the fees are very fair. Pricing mark-up isn't as crazy as people think and the labor rates are insanely cheap if you really think about it.
For example. An average pair of speakers takes about 1.5 hours to install correctly. Most places charge around $50 to install a pair of speakers. So when you break it down, thats roughly $35 per hour. Show me one garage with labor rates that low. Car audio is one of those areas that you really do get what you pay for. If you cheap out, you WILL regret it.
Please don't get me wrong, I'm not trying to make you change your mind, but I have seen it time and time again where people think their friend / buddy who has done a ton of installs is better because they are cheaper and then end up paying twice the cost to have it all re-done correctly; or worse end up having to pay out hundreds of dollars when components of the vehicle get damaged from an improper install. It may not seem like much, but I've seen a pair of speakers cause a complete window motor failure that ended up costing over $200... so the $50 install fee didn't seem that bad then. And don't even get me started on an amp install. You are dealing with a lot of power through that main power wire and an improper install can and more than likely will cause the vehicle to catch on fire... I've seen it. Also, make sure to use quality parts. cheap parts WILL fail and at the very least leave you with a non-working system... or worse, leaving you watching your beloved car burn to the ground because the fuse didn't work as it should.
Yes I own my own installation shop, but this is coming from me as a fellow Subaru enthusiast just trying to give advice. Whether you decide to do it yourself, go to best buy, or come to me really doesn't matter to me... I just don't want to see anyone getting screwed or screwing themselves over.