That's a feed from your running lights, so a brake controller can have an illuminated dial that comes on with the car's lights.NOTE::: There is an extra pin in the trailer brake socket coming from the vehicle that the trailer brake controller doesn’t use. I don’t know what this is, but it doesn’t seem to hinder function. Please enlighten me if you know what that’s all about. I haven’t investigated further.
Nice install by the way, although I don't think I would have been brave enough to attack the plastic trim as aggressively as you did!