I have not used one. But I looked it up and from what I can tell it is normally closed. This would mean that the lights would stay on all the time and then when you triggered it, they would go off for however long the timer is set for. I am not positive but that is how the description reads to me. Also, if there is a normally open function (which most relays are) it says it requires a pulse or a change in state. You would get a pulse if it hooked into the door locks instead of the interior lights. Also, if you happen to turn your dome light on while driving you would turn on the underbody lights if hooked up to interior lights. But if your truck automatically locks the doors once you start driving then they would come on again when this happened. You might want to try and find a forum that deals with alarm installs as that seems to be where it is mainly used from what I could find. Good luck!