"Trailer Disconnected" despite lights working

[Tomcom21]

Where would the leads from the multimeter go to test the resistance ? From the 7 pin blue to where ?

 

[persistent]

Feel free to start your own thread. This one is too old for good response.
Post year, make, model of tow vehicle.
Post year, make, model of trailer.
Post symptoms.
Do these work:
Tail/brake lights
Backup lights