25%

I have one speaker installed, wired, working, and buttoned up.
Next up is the left side rear door.
Here's my wisdom to people doing a similar phased install.

• Run your speaker wire through the door at this point, leave about 1' in the door and maybe 12-20' wrapped up in the pillar cover for later. I think that it might make sense to run the passenger side speaker wire, to tie into the amp, back and across the rear seat rather than forward and under the dash. There's less clutter to deal with - at least the rear speaker.
• I used the 5-year foam gasket from le home depot. I cut 1" strips and stuck them on the door and complimentary 1" strips and stuck them on the spacer. Together they make for a good seal. The question is how this solution will age. 
• In the passenger rear door, the red/white wire is negative (!!) the green grey wire is positive. Thanks to this link.

When I do the install for rear door #2 I'll include pictures.

Running wires and fitting spacers

Last night I figured out which way the spacers mate to the door. They are not symmetrical, so you can't tell without aligning them to the door and checking that the holes line up.

When securing the spacers to the door, you want the surface between the spacer and the door to be sealed such that no air escapes and there are no loose surfaces to rattle. You can use weatherstripping foam, in a roll, adhesive backed to seal the surface. It looks like I will make a circle of foam on the spacer and a circle of foam on the door. the foam compresses very well so I think this plan with create a relatively tight seal. I could probably seal the gap closer to 100% with silicon caulk, but I'd rather do the less obtrusive tape for now.

After test fitting the spacer, I passed a length of speaker wire through the rubber channel that goes between the car pillar and the door. There are already OEM speaker wires and power window wires in this channel, so I had to add the speaker wire. I figured to do this now so that I won't have to mess with running a speaker wire in a hard spot when the spacer and speaker are already installed. I'll switch over to the speaker wire when I install the amp in phase two, but for now the wire can just hang.

Finally I pre-drilled the speaker to the on spacer. I'm going to use 4 x 8-1/2 sheet metal screws  to secure the speaker to the spacer. The speaker itself came with 8- 1/4 (or maybe even 8-1/8" sheet metal screws) - tiny screws. they will work too and I might implement them if I don't get to the store to get the 1/2" length. Actually I believe that by anchoring the screws a full 1/2" into the plastic, it will strain the spacer less.

So I did a lot of fitment. I think I'll take more explicit pictures when I wire up the second rear door, after I figured out and executed the process once.

Spacers?

I purchased my spacers for subaruaudio.net. I requested spacers for a 2012 NON-WRX impreza, and submitted the speakers I was planning to install. After a quick paypal payment, the package arrived very promptly.
Since the install is ongoing, I can't comment on how the spacers fit into the process, but so far so good.
Here's a web page that's relevant to my project: http://www.subaruaudio.net/installations/2012-impreza-non-wrxsti/

Other rear door pictures

1. When removing the rear door panel, note the tab you have to depress to pull the plug out





2.  This is the diameter of the speaker cut out in the door. About 5 1/16".





3. Here's the distance between the two lower speaker frame screw points.About 5 5/8". The distance to the other hole above these was further than my 6" caliper could reach.





4.  Here's a straight on shot of the door with speaker removed. Notice the contours of the door panel. Sealing between the spacer and the door is going to be a challenge.





5.  Here's a side shot of the spacer mated to the door. Note the large gap that we'll have to seal.





6.  I believe that gap is about 1/4". I think I'm going to add foam gasket from both the door and the spacer.

Rear Speaker Measurements

Rear speaker in mount removed from door. Distance between bottom of mounting flange to top of speaker frame. Just under 1.5".

 Measure from bottom of mounting flange to furthest point of speaker. Looks like 2".

So you could bring your speakers forward about about an inch without any worry, probably as much as 1 3/8". The spacers I had made for me were 3/4".
So with my JBL P662s, and the 3/4" spacer, I have about 1/4" more clearance on the window side and 1/8" more clearance on the door panel side.

What wire?

People argue over what size wire you should buy for any application you can think of. Companies want to sell you over-spec'd high-margin products. I believe in buying as much as you need and no more.Both your speaker wire and amp power wire should basically let the current flow and restrict it minimally. Too much current restriction and the wire heats up and can cause component failures (amp) or attenuate the sound (speakers).

Speakers:
This link has a wonderful, lengthy discussion about speaker wire. My take away is that 16 gauge should be fine. I think that 18 would be OK also. I might do 18 to make it easier to run through existing channels. 

Amp:
The manual for the Alpine amp says to use 8 AWG (gauge) power wire, so there you go. I was looking for an Input Current spec as a check, but that seems to be unavailable. 8 AWG sounds like the right thing to do.

Major components

• JBL P662 coaxial speakers for rear
• JBL P660C Component speakers for front
• Alpine MRP-F300 amplifier

High level task order

Essentially you can install the amp or speakers first.

If you install the amp first you get more volume from your stock speakers and you complete the hard tasks earlier. Then when you add the speakers, there's more of a reveal - Amp first is delayed gratification.

If you install the speakers first there's a more noticeable difference right away, but adding the amplifier second puts the harder task at the end of the project- I've always like to get the hard stuff done first.

Speakers first task order:

1. Install front speakers
2. Install rear speakers
3. Wire speakers into existing HU
4. Install amplifier power cable
5. Install amplifier
6. Wire amplifier to HU
7. Wire speakers to amplifier

Amp first task order:

1. Install amplifier power cable
2. Install amplifier
3. Wire amplifier to HU
4. Wire speakers to amplifier
5. Install front speakers (including wiring)
6. Install rear speakers (including wiring)

I have decided to install the speakers first.

The project: Upgrade car stereo

Because I love music, love DIY, spend about an hour in my car daily, and like hands on projects, I thought to upgrade the stereo in my car. When car shopping in 2012 I considered all of the premium sound systems, and none of them were that great. Common knowledge is that you get much more bang for your buck when you purchase and install aftermarket stereo components in your car. So this project represents many things that I love coming together.