From what i have noticed, if it is not dark enough for the headlight symbol to illuminate on the instrament panel in "auto".......then in order to have the tail lights on you must turn the switch to "on"........as you have mentioned. DRL's only turn the inside lights on in a reduced brightness mode. If you want to have the tail lights on in this situation then the "on" position is your only option. I have used that several times when it is slightly dark but not enough to turn on the headlights. Adjusting the brightness might change the operation of the system.