Grounds were in good shape - so I went looking elsewhere.

As I mentioned, right headlight was dim. So I pulled it, checked the voltage. 11.9 volts. And I noticed that the wipers weren't running. Plugged the headlight back in. Dim light, wipers go on.

Pulled the plug and put a test light on the plug, both pos and ground at the plug. Light comes on like sunshine, wipers stay put.

Plug the offending headlight on the other side. Bright as can be.

So, with nothing better to do, I reach down and move all the wires around, and notice a plug, black, two poles, without a home. It's travelling in the same bundle as the headlight wiring. I pull it out of the cover as far as I can.

Plug the headlight back in, comes on bright as can be, and, best of all, no wipers. I cable tied the unknown plug back on the wire cover, put every thing back where it belongs, and everything works like it should.

Since it is now not broken any more, I'm not going to fix it any more. Best guess? That the unknown plug was wiring for the never installed headlight wipers, that there was an insulation failure inside the wire cover, and that created a positive short that then created a feedback completed circuit to the windshield wipers.

Or something like that.