Understanding the running light switch

[jonathan-m]

I've been battling rear light issues for years now. I'm off this week, so I'm determined to kick them in the butt once and for all. I've run 10 gauge wire from the tail light housing to a grounding point on the frame, which itself is connected via a 10 gauge wire to a grounding point on the on the frame in the engine bay that is then ground with a zero gauge wire to the engine block ground. (Thanks to Markeb01 for the grounding idea)

I messed around with it a lot today and got the brake lights working and signals working properly. I replaced the flasher and I bought a new rear harness as well, which I will install tomorrow.
The one thing that I still have not gotten worked out is the running lights. As soon as I pull the headlight/running light switch the headlights function but nothing happens with the tail lights. If I hit the brakes with the switch pulled the brake lights function as normal, as do the turn signals.
I haven't been able to isolate what would cause this, so I was hoping someone would be able to point me in the right direction.

Thanks guys!

Jon