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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.