So there we were on our way back from AZ where we picked up our 13' Whaler. The trip back was long but driving conditions were good until we got to Cabbage Hill in Oregon. We had fog like I have not seen in many years. Cabbage Hill is a 6 mile long 6% grade that winds it way across the face of a couple of pretty big hills.
In good weather it's a 50-55mph drive because you don't want your momentum to overrun your ability to use the brakes to slow down.
So there we were in the fog, driving about 25-28mph with my emergency flashers on. My wife was watching all the signs and reading them to me because I didn't want to take my eyes off the road or my rear view mirror. A few cars came up behind us and that's OK because they had their headlights on. But then I noticed the first one didn't have any taillights. Or the second one or the third one, etc.
That got my feeble old brain to wondering why so many cars with their headlights on didn't have any taillights. Then it hit me--when you are driving with your daytime running lights (DRL) on, only your headlights are on, but no lights in back.
I checked my GMC Terrain and sure enough, with the DRL on you have no taillights. Neither does the trailer. It's only when I turn on my headlights (using the switch on the blinker arm) do my taillights work.
That's important to know in situations like heavy rain, snow, fog, dusk, dawn, etc. You might want to check your rigs and see if they're the same.