New rig, could use some troubleshooting advice.

Brake lights: take the cover off, take the bulb out. Put one end of your meter tester in the center of the socket. Bring a good ground from the battery to where you are testing. You might be getting the power, but no ground return.

Before you try to determine how power gets there, actually see if power is there. It you determine there is no power, bring power to the positive side of the bulb, and prove it works when you have good power.