I'll do what you suggested on the rear harness although I've been assuming all that must be fine because the blinkers work. The blinkers use the same circuit on the rear wiring harness as the brakelights. I suppose it's possible than when the blinker is on it's getting some kind of ground through the front turn signal that is not available when only the brakes are pressed. (But I didn't see anything that looked like that I looked at the diagram.)

I've verified power all the way up where it comes in to the turn signal connector, and all is good.

Here's the rest of the diagram. Thanks for any help you can provide.