I think the head light switch is the easiest to get to. Pull it out, then check for voltage on the red wire. If it is not there, we will have to go back to the fuse box. If it is there, check for 12 volts on the light green wire when the switch is turned on to the headlight position. To be of value, the testing has to be done when the problem is occurring. When it is intermittent like this, I fashion a test light or I use a clip lead to keep the voltmeter attached to the terminal, then I can quickly look at the light or meter when the problem happens again.
My suspicion is you are going to find the voltage goes away on only the light green wire. That is because there is an auto-resetting circuit breaker inside the switch assembly. Those trip based on temperature, and pitted contacts on the breaker cause heat to build up. Those circuit breakers cause more problems than they prevent. An additional clue is when that circuit breaker trips, the tail lights / running lights will still work because they're on a different circuit.
Saturday, March 19th, 2016 AT 3:22 AM