I just got my first delivery from partexpress.com today. Yes, the service was fine. Not just that, but they have an interesting mix of stuff in their catalog -- I just finished 30 minutes flipping through it.

Phoenix42, Like Doug says, sounds like you really do have a plan. Yes, running speaker wires back up to that harness adapter lets you use the factory harness with no cutting (I just couldn't get fired up about pulling separate 10G wire into the car doors!) . Soldering to the harness adapter is fine. I took a different route and used shrouded spade connectors (from good old Radio Shack) reinforced with heat shrink tubing. They have worked fine-plenty of friction in the connectors. To reduce chances of reverse polarities, I used the male connectors on the postive/feed side of 2 wire connections like power, speakers. If you go the crimp-on shrounded spade route, just make sure to use a good ratcheted lug crimp tool (partexpress has a few for under $30) -- the non-ratchet, cheaper type don't put enough pressure on the crimps so might lead to a failure.

Lastly, if you're using a seat bolt ground, just be a little extra careful to make sure you have a good, clean metal-to-metal connection.