Google some installs and you will see discussion of the harness. Also, the install manual may be online.
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Harnesses from theretrofitsource.com are the highest quality I've ever run into, they do what you guys have been explaining and I run two of them in my Volvo and one in the Expo. They even come plug and play as seen here: https://www.theretrofitsource.com/components/harnesses.html
I am in no way affiliated but feel the need to rep a good product when I see/use one.
Oh, they're $30!