I put it into gear and it dies.

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Ok, I just rigged this up real quick and there was no light anywhere except one VERY specific spot in the crank position right before the starter kicked in where the light was super bright on the "input" side of the ballast and dimmer on the "output" side. Other than that, nothing.
Ok cool. You verified you have a full 12v in crank at least and that's correct. Now you need to start back probing until you find power in the run circuit and see "where" that break is occurring. Kudos for that rig. That was smart.