Is the trucks "trailer sensor" preventing this 7 pin tester from working?

[Viking Truck]

Im working on adding a 7/4 pin trailer connector in the bed. To check my work I bought this tester but I can only get the 12v light to come on when I test the factory 7 pin on the bumper, not the r/l turn/stop or running lights. Of course I want to make sure the tester works on a known good connector before testing it on the new. It works on my Jeep. Is the truck computer not sensing a trailer and preventing me from being able to test?

 

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[gemarsh]

Im working on adding a 7/4 pin trailer connector in the bed. To check my work I bought this tester but I can only get the 12v light to come on when I test the factory 7 pin on the bumper, not the r/l turn/stop or running lights. Of course I want to make sure the tester works on a known good connector before testing it on the new. It works on my Jeep. Is the truck computer not sensing a trailer and preventing me from being able to test?

View attachment 30437

Yep.

There is a gm tester, but daaamn it's expensive!