Wow... this got off track...
OP, did you ever get it figured out? Someone brought up a good point though, DRL will only send a signal to your headlights, not your trailer running lights.
I still think you should try my idea...turn your light switch to AUTO and place a piece of tape over the light sensor. All your lights on, all the time. That provides power to your running lights, therefore a signal that you can use to power your camera. Seems to hit the mark for safety, convenience and capability.
Are there anymore cars on this train track that can be derailed...?