"Truck has older restoration with 12V conversion", so not sure what's what for electrical. Looks like I see an alternator already wired? If so, just copy with new wire. Sounds like you are pretty much satisfied and will be installing harness soon. That's all good. Assume you checked the fuses for the components that don't work. Not sure if you need an add on harness at this point.

The black box is an Echlin VR32 external Voltage regulator. I assume because the alternator does not have internal.

Also, I believe the small gray can on left is a relay.

Left of relay may be an inline fuse?

It may help to mark and remove old harness in "one" piece. Then if there are any questions you can gently open it up by taking off sleeve/old tape and following wire. Mark all ends and describe to yourself where they were attached or make a drawing or take pix.


Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn