Tail light trouble

Hey everyone,
I have a 2010 Tacoma with LED tail lights. The left tail light works fine except for the running light. The brake, reverse, and turn all work. I have plugged the left tail light into the right tail light socket and it works fine. I plugged the right tail light into the left socket and the running light didn't work.
After this simple test I concluded that the problem is in the wiring...somewhere. There is an aftermarket towing kit, but the problem persists even after bypassing the towing kit (the kit did not require cutting any wires....all plug in). The fuses I have found are all o.k (located under hood and behind cubby on left side of steering wheel). Any ideas?