I have a 97 F-150 extended Cab.

On the inside brake trailer harness there is a blue wire that I thought went
directly back to the 7 way jack. However there is a slight voltage
(fluctuates around 1v). I was hoping to use this with a switch and send the
power from the red wire back to the blue brake wire on the trailer harness.

Should I be doing this? Where does the voltage come from on the blue wire?

I am hoping to put some lights on the back of my truck and this would be a
great way to turn them off and on without having to run more wires. Also
the red on the trailer brake power is hot all the time where the aux charge
power on the rear is only on when the key is on. If I can use the trailer
brake power then I can have it on without the extra drain of the
accessories.

Thanks!