What to do when the lights go out.  Easy headlight switch troubleshooting and why they go bad.

If you're driving a classic Mustang regularly, you've undoubtedly experienced flashing headlights and/or flickering instrument lights.  There are reasons for both and ways to correct each without having to call an automotive electrician.

Instrument lights flicker from a dirty or corroded variable resistor (the instrument light dimmer).  Visit your nearest electronic parts store, such as Radio Shack, and buy a spray can of tuner cleaner.  Tuner cleaner has a very high evaporation rate, like brake cleaner.  Blast the resistor coil around the headlight switch shaft (embedded in ceramic) and cycle it back and forth from dark to bright.  Allow to dry.  Turn on your parking/headlights and try it out.  It should function normally.  If it doesn't, replace the switch.

Headlights cycle off and on whenever there's a short circuit or circuit breaker contacts inside the switch are badly corroded or dirty.  Dirty/corroded contacts cause resistance and heat.  With heat, the circuit breaker does just what it's supposed to do, it disables the headlights.  But, it doesn't completely disable them for your safety.  It cycles them off and on.  Cycling headlights aren't always a defective switch, but instead a real short circuit between the headlight switch and headlights.  Search for potential trouble spots, such as insulation worn through to the wiring and evidence of a short circuit.  When there's a solid fault to ground (a short), headlights generally will not work at all.  The circuit breaker will fault (open) and disable headlights.  Most of the time, flashing headlights are dirty/corroded circuit breaker contact points inside the switch.  Some daring folks can open the switch and clean those contacts.  Most of the time, it is advisable to replace the switch.

Headlight switch removal is simple.  Turn the knob all the way counterclockwise, depress the tiny knob on the switch, and pull the headlight switch knob/shaft all the way out for replacement.  Always check the plug and wiring for good serviceability - no broken wires or dirty contacts.  Clean those contacts with tuner cleaner.