With the test bulb in place, was it full brightness? If it was, what happened when you moved the signal switch?
The head light switch shouldn't be involved with the brake lights, but it is with the tail lights. Head light switches, dimmer switches, ignition switches, and heater fan speed switches are all in high-current circuits, and as such, it is common to find overheated connector terminals with the connector bodies melted in those areas. In addition, head light switches have internal thermal circuit breakers that cause more trouble than they prevent.
With all of these switches, a little resistance can develop between a pair of mating terminals, or between a pair of contacts inside that switch. That resistance results in heat being generated when current flows through them, and that heat causes more resistance to build up. That progresses until the terminals or the contacts overheat. That heat migrates from the contacts or the terminals to the other one, so there's always two parts to the repair. The switch must be replaced, and the burned terminals must be cut out of the connector body to be replaced individually, along with four inches of their wires that will be hardened from being overheated, and solder won't adhere to them.
The thermal circuit breaker in the head light switch develops the same arced contacts that cause heat to be generated, and that heat causes the circuit breaker to trip, then automatically reset about a minute later. That defect is usually limited to the switch itself and doesn't involve the connector terminals.
Regardless, it would be a good idea to inspect the switch terminals and those in the connector, for signs of being overheated. Also look for any wires that have their insulation melted in that area.
Friday, January 24th, 2020 AT 4:26 PM