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Old July 13th, 2011, 12:53 PM
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Jason Cohen
1994/Defender 90
Join Date: Mar 2011
Location: Seattle, WA, USA
Posts: 2
Trailer Wiring Harness

Iím finally getting around to diagnosing why my trailer brake lights have stopped working. Long story short, looks like the little black box that converts the brake light to the two turn lights for the flat 4 pin is done for. While considering replacement options, I noticed there is a nice 7 pin round plug under there that this box plugs into. So I figure I might as well put in a full 7 pin socket in case I ever want aux power on my trailer. Great theory, but Iím having trouble locating aux power and reverse light out of the seven pins. I also noticed there are two other pins coming from that same bundle that are currently plugged off. So this is what I know so far (see picture for plug location of wires):

7 round:
black = ground
green / red = left signal
green / white = right signal
green / purple = brake
red / black = running / tail
red / yellow = ?
red / orange = ?

2 Extra:
purple = ?
green / brown = ?

After searching the forums a while, it looks like purple should be aux, but I canít get any signal from it. I had the ignition on and every light turned on, car in reverse, and still nothing for any of the ? above. I checked the fuses in the engine compartment and everything looks fine. I also have no idea what those other two wires are in the plug are supposed to be (maybe fog light)? I found some info about a 12N and 12S plug, but those have a mix of male / female, whereas my plug has all the same on each side.

So hopefully this thread can serve as a useful summary of some of the information people might be looking for. Also, if anyone knows for certain where aux power and reverse lights are supposed to be, that would help me troubleshoot. Otherwise my next step is to take off the rear speakers and start tracing the wires back further. Worse case, if I can find aux power somewhere back there, I can just run new wires from there and over to the reverse lamp on the back. If it turns out to be too much trouble, Iíll just wire up for a 4 wire setup and deal with it if I ever get a trailer that needs more.

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Old July 13th, 2011, 07:24 PM
Antichrist's Avatar
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Tom Rowe
Join Date: Feb 2006
Location: Atlanta, GA USA
Posts: 1,287
This is what I did for my Discovery, would work as well on a Defender.

I've since replaced the crappy plastic Hopkins trailer socket (after breaking two of them) with a good Cole Hersee 7-pin metal socket.

Tom Rowe
Atlanta, GA

Four wheel drive allows you to get stuck
in places even more inaccessible.

62 88 Regular
67 109 6cyl NADA x2
74 Lightweight - The Antichrist
95 DI 5-speed
95 D90 5-speed
97 D1 Automatic
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Old July 13th, 2011, 07:53 PM
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Capt Milks
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Defender V-8, Defender 2.25
Join Date: Oct 2004
Location: United States
Posts: 1,712
Purple constant hot
Red-Yellow fog lamp switch to fog lamp
Red-Orange Fusebox to rear fog guard switch
Green Brown switch to reverse lamp

The purple would be used to provide the power to the trailer.
Land Rover 110
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Old July 13th, 2011, 09:33 PM
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Jason Cohen
1994/Defender 90
Join Date: Mar 2011
Location: Seattle, WA, USA
Posts: 2
Thanks for the info, super helpful. Now I know what it should be, just need to figure out why the constant power and reverse lights don't work.
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