Why are the headlights getting dimmer and brighter?

Alternator And Battery Should Be Checked. Auto Parts Store Should Check This Stuff For Free, you May Have To Take Part Off But They Will Test It For You. Just Pull Up To The Store And Ask Them If They Can Test Them To See If You Should Replace One Of The Parts. Could Also Be A Loose Ground. Check Your Connections To Your Battery And Make Sure They Are Tight. (Make Sure They Are Clean With No Corrosion) Check Connection To Your Alternator Also. Make Sure The Grounds To Your Head Lights Are Grounded Securely To The Body Or Frame Of Vehicle.