Electrical wiring Trailer/Cab

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Question on 7 flat pin wiring.
Trailer lights, brakes have been working. I would like to hook up backup light circuit and 12v circuit for in cab switch for under trailer spotlights.

When plug is not connected, I have 12vdc on correct pin,
When plug is connected no 12vdc at that wire in junction box.
No extra batt.or breakaway is installed.

How do I access the relays/control wire in cab?
Suggestions, direction, links that work, pics, diagrams, etc.,
I would like to gather info see what I should be seeing.

In advance, Thanks for any info.

[Schnot]

So with the plug plugged in you don't have power in the junction box on the trailer?

Sorry but how you wrote it is hard to follow.


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[Schnot]

Also all the fuses for the trailer connection are under the hood on my 01's.


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[Ad Exsp7]

Thanks for the reply.
Basically the 12v at the 7 pin is 0v once the plug is installed.

The actual topic is that I need to find out where this is accessable in the cab.
What panel, what pins, where to intercept, The control voltage for the 12v supply for that pin, is what I would like to switch via relay. Also the reverse Circuit needs to be found.
Actual wire color codes, component pin numbers etc,
Owners manuel does not have it that I have seen.
Access to Ford's actual schematics would be nice.

Thanks in advance.