blue and red wires under driverís seat
On my 2017 chassis/2019 Forester MBS, the blue wire goes directly to the electric brake blade of the 7-blade receptacle, without fuse. The red wire under the driverís seat is power and is fused (20 Amp in-line fuse holder accessible through the side panel of seat base). The red wire powers the Hopkins (Curt on some models) light controller located along the driverís side frame near the rear bumper, and is split before the controller to run power to the power blade of 7-blade receptacle. ALL tail lights are controlled by the Hopkins controller to isolate the Mercedes chassis wiring from the coach and trailer wiring and lights. If the in-line fuse on the red wire blows, YOU WILL HAVE NO TAIL LIGHTS. Also if the Hopkins controller malfunctions you will get weird results. After a Hopkins unit failure, I had the left side brake light on full-time even with the key off (the red wire powering the controller is always hot), and no other tail lights. I believe that there was a wire end connector on the blue wire that was supposed to be compatible with certain electric brake controllers, but I ended up removing it and wiring my electric brake controller (for boat trailer) and dash-mounted feedback indicator from my Stay-in-play Duo (for towed car) myself, using the blue wire, a diode, and a power switch. I also ran a feedback wire from a tail light to a relay wired to the headlight switch driving a dash-mounted LED indicator to light up if the headlight switch is on and the tail light is off at the same time. I felt this was a necessary safeguard after losing my rear lights twice due to blowing the red wire fuse and a malfunction of the Hopkins controller ó all eventually found likely to be caused by improper installation of diodes in the brake wires of my towed car by the mechanic who installed the stay-in-play.