The mystery ELT wire was solved when I removed the unit and found that it has been buried underneath it. So I connected the wire to the harness and reinstalled, then used a couple of tie wraps to secure all of the extra wire to the harness.
The lighting wire presented another mystery. First, I drilled a #12 on the underside of the rollbar, then ran the two wires (a white one and purple/brown one; L6028 (YEL/PRP) and GARMIN: F7139 (WHT)] But the plans say there's a twisted pair with L6029 (YEL/GRN). I have no idea where that one is but it's not in the harness as near as I can tell. I'll figure it out after Oshkosh.
And I'll have time to study because the plans call for a snap bushing -- SB 187-2 -- which appears to have been shipped with the finishing kit. I found the bag it is alleged to have come in. No snap bushing. Do I REALLY want to pay shipping to have one stinking snap bushing sent from Van'. No, I do not. I'll look for one at Oshkosh. But I can't do any more work on 42MiS/U-27 until these issues are resolved.