I need some help if anyone knows how these work? It is on a Cayenne.
I would like to use it for two things, A aux brake light and to tow a small boat.
First, I have a cool 3rd brake light for the hitch that I would like to tap into this I found the ground but the brake light terminal seems to be sending a signal and not a constant 12v.
It seems like there is only one terminal that sends a constant 12v.
I am stumped on how this plug works?
Thanks and happy Friday !
My stupid over-engineered-under-built Mercedes tried to be really smart with its 7-pin connector. It wouldn't just send voltage down the damned wire - first it had to check and see if there was anything attached and what the resistance was and IF and ONLY IF it determined that there were the correct number of lightbulbs and those bulbs were up to its standards would it then send voltage.
I suspect Zee Germans may have done something similar to you.
Use diagrams to verify the Cayenne is similar before doing anything, it may have some wonky German-European thing going on.
The sailplanes we buy from Germany have weirdo electricals and yellow-orange-red tail lights. 
wae said:My stupid over-engineered-under-built Mercedes tried to be really smart with its 7-pin connector. It wouldn't just send voltage down the damned wire - first it had to check and see if there was anything attached and what the resistance was and IF and ONLY IF it determined that there were the correct number of lightbulbs and those bulbs were up to its standards would it then send voltage.
I suspect Zee Germans may have done something similar to you.
that would not surprise me. My disco has a light on the dash that comes on when you plug a trailer in. It works well if you have the resistance of several incandescent lamps in the circuit. It does not light up if you plug a trailer in that only uses LEDs. Thankfully it does not seem to affect how the trailer lights work, it just never illuminates the warning light on the dash.
Ah yes, thank you guys.
Trailer wiring in the US doesn't have a dedicated brake light wire unless you get up to the 7-pin round pin semi plug. In the US there is a left brake and right brake, and they both flash when the appropriate turn signal is on.
The 7-way flat contact rv plug does have a contact for electric trailer brakes, though. If you don't ever pull a trailer with trailer brakes, and you don't have a trailer brake controller installed, you might be able to wire up the trailer brake controller pigtail so that the brake input signal goes straight to the output signal or activatrs a relay, and that would give you a dedicated brake light signal at the plug. Doing that, though, you would want to have a very low amperage fuse inline so that just in case someone hooks up a trailer with brakes it would pop the fuse instead of locking up the trailer brakes.
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