What is Unused 5th Wire of Brake Controller Wiring for

Question:

Im interested in a trailer brake controller but noticed all that I am finding use only 4 of the 5 wires on the vehicle side of the connector. What is that 5th unused wire for? And are there any brake controllers that use that 5th wire for an additional feature?

asked by: Mark

Expert Reply:

Typically the 5th wire of a brake controller wiring bundle on a vehicle is the "illumination circuit" which is basically a circuit that activates when the headlights of the vehicle are on to power a backlight for the brake controller. This is not typically used anymore since brake controllers have digital displays now so it's not needed.

expert reply by: Jameson C