I think it's a "Run take off" source for power. If you remove the left side footwell trim there is a dedicated customer use wire hidden away behind the harness (there are actually several, one is for ignition switched power and the others are for various lighting when the truck is upfitted). The switched ignition source is fused at 10 amps and is intended to be used to power relays that will switch your accessory loads that you run battery power to, it is not intended to directly power accessories.
You want the green one with the white stripe.
Thats the one I was thinking of, thanks!
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