2006 GMC Savana Running lights

When I turn my van off my running lights stay on so I have to unhook my battery every night. Sometimes if the the van has been sitting and the lights are out they will come on while the van had just been sitting. I've tried a new flasher which is costly but that wasn't the problem can you help. I hate guessing games and that's whats happening everyone guessing what it is I buy the part to find out later it wasn't it