I'm a bit confused. (That's normal, though.)
Are you seeing 12v constant on the right turn signal post on the truck? Is the blinker blinking when this test is done?
Chevy has been selling trucks with the "traileribg package" for years, but never hook up the constant 12v post. So, there should be a constant 12v post, but not at a turn signal. You'll have to connect that lead for the 12v constant line to the fuse box under the hood. (Assuming your truck is a chevy)