The emergency flasher switch is the likely culprit if all your bulbs don’t light up. It connects power (through a flasher) to the brake lights and the front turn signals. It is a four wire switch and it shorts all four together when you turn it on.
Aha, thank you. I hadn't got any reaction out of the emergency flashers until this recent troubleshooting so the switch could well be worn out/oxidized, it jumped into life after excercising it a bit; I'll get one ordered.
