To test the switch... just flip switch and use a test light to see if power hits the middle posts... I believe the bottom posts are the hot leads middle posts are low beam... and upper posts are high beams... My bet is FAULTY SWITCH!!! Others as well as myself have had switch issues... my switch happened to be a window switch...
Good luck
The entire panel that the light switch is in pops out.... allowing easy access to the back of that switch...