the box of this scanner says that the yellow should be attached to alarm's output or ground.
so it's saying attach it to either a ground or power source??? I'd say put down that backwards manual. lol
scan the page of the manual cause maybe you are reading it wrong.
typically like mentioned in the first post, yellow is constant. but ya never know. they gave good advice. I suggest trying it out.
to fiond the output wire just look for the little blinking LED. The wire connecte to that is the one you need