Thanks, jrelliott. I had heard this could be done. I find there are a couple places you can buy it premade -
https://www.speedwaymotors.com/Speed...xoCgREQAvD_BwE
https://www.just8ns.com/DI1000-Years...hoCJW0QAvD_BwE
Doesn't do me much good as my alt is 6V positive ground, and speedway says not for one-wire anyway.