Good news! I figured out the electrical problem on the ebike!
Bad news! The connector is nonsensically located inside the bike frame.
And I will be damned if I can figure out how they ever fished it in there in the first place, since the wire on one side is potted to the charge controller and the wire on the other side is part of a bundle that couldn’t possibly fit through that space. I’m missing something obviously, I mean they did do it so probably I can too.
Once I removed the grommet at the bottom of the frame, a little very light tugging made it immediately obvious that something that should be attached … wasn’t. When I took my relatively high-speed header in the mud on Tuesday, the front fork spun in a most un-ebike manner and a couple of minor connectors were pulled loose at the handlebars. So another disconnected connector didn’t surprise me, though I couldn’t find it until I plumbed the depths of the frame itself. I never would have been able to firmly diagnose the problem if Radrover hadn’t sent me all those free parts two years ago including a replacement harness. Once I had everything electrically connected, the display and motor worked perfectly. So that’s good.
But right now it’s just an oversized bicycle with a whole bunch of wires hanging everywhere. Getting it properly back together is going to take some patient thought. Still, I feel much better about the whole thing knowing I can fix it one way or another.