Ok, I figured out the problem. I pulled off the cover to the rear light, and looked at the bulb. It looked like one of the two filaments was burned out. I popped in a new bulb, and suddenly the problem went away! Watching it closely, when the headlights are on, and the rear light is on, once you start the blinker, the two filaments toggle on and off, one right after each other. One is just slightly brighter than the other I think. So, what I think happened was this. Normally when the lamp fails, you will get the fast blink signal on your dashboard. Since there were two filaments, and the one that was burned out wasn't used until the headlights were turned on, the blinker would blink normally until the headlights were on, and then it would sense that the lamp was out.
2012 Routan SEL