My wife has a 2005 Mercury Grand Marquis. With the light switch in either the Auto or On position the headlights do not turn on. However if she pulls the Bright/Dim switch and holds it tight towards her, the headlights come on. A local mechanic told her to replace the lights, but I cannot see all of them (bright and dim) going bad at once. I am thinking either the switch or Light Control Module. Is there a way to test these without blindly replacing one just to find out I guessed wrong?