According to this email response from WARN:
"On the older 3 wire system the white was the hot feed to the remote and the green was power-in activation while the black was power-out activation."
and the dwg below - that was labeled "WARN 8000 control circuit" (and also included a footnote stating "WARN 8000 remote control circuit shown, but all WARN remotes use the same wiring":
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So, it would seem that the white lead will be the hot wire.
As a bonus, I stumbled across this homemade diagram. It was done by a guy who had just successfully completed the modification you are looking to do:
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It looks like a valid wiring diagram to me. I especially appreciated the pilot light - to alert the operator that the system was powered up.
Finally, and I hope I don't sound preachy; When you are up to your asshole in alligators, you'll want this system to operate as designed. The success/failure and dependability of this modification will mostly hinge on the attention to detail paid during the initial fabrication. It will take more time and money, but be sure to use good engineering/electrical wiring practices like:
Using the correct gauge conductors for the new wiring runs (if necessary, spec the next biggest AWG size to accommodate for the increase in length)
Properly routing, sheathing and supporting the added cable runs - also, use rubber grommets at any steel penetration points.
Securely fastening and weather-proofing any associated mechanical components (e.g control boxes, switches, lamp bases, etc)
Using soldered
ring - not fork terminals at any wire terminations/connections/splices (try to avoid the use of solderless, crimped-on connectors).
Using lock washers at any threaded terminal connectors.
Good luck. I am sure you'll do a professional installation. Please show us some images when you finish. I am planning a similar system for my truck. Also, I am considering taking it one step further - interfacing a wireless remote. Check this out:
http://www.colorado4x4.org/vbb/show...-you-holler-Wireless-winch-control-setup-Warn