Taillight wiring, where does yellow wire go

[bmklawt]

Have some interior panels off and was looking at the taillights and notice I have a yellow wire in the third can from right that goes nowhere, the taillight on the other side is the same, this can has a dual filament bulb in it, is this suppose to go somewhere?
On both sets of taillights the end of the yellow wire has no connector on it, is corroded and does no look like it was ever used.

If it is not used can I hook up the brake lights to this for better visibility for the person behind me.

[jimmini]

On a 7 way trailer light plug yellow is the back up lights.
Dose your trailer use a 7way plug?Some older plugs where 5 way.

[Over59]

You should already have brake lights. Back up lights would be cool since you have it apart, maybe one of those irritating buzzer things.

[bmklawt]

The first light on the left looking a picture is the the turn/brake light, second is back up light, third the one with the yellow wire is a running light I assume, fourth is a clearance light, fifth is a reflector.
It does not look like the yellow wire was ever used, I just want to make sure of that.