Apparently you need one for trailers over 750kg.......I think they operate the electric brakes to help you stop.
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Well I can't affords a trailer and never tow one with electric brakes so mine just gets in the way !
Tekonsha voyager,top of dash to right of steering wheel,2000 130.
I mounted mine next to the aircon button. The aircon button was moved from the middle to the far left and the electric break light was fitted to the middle button and the electric break controller was fitted next to it.
Chris