once you have checked the voltages it can realy only be the 1 wire from the switch to the light cluster plug
although you have tested the switch with a multimeter maybe remove the plug on the switch link the 2 wires with a bit of spare wire turn ignition on see if brake lights are on? switch maybe dodgey under load?
although you have tested the switch with a multimeter maybe remove the plug on the switch link the 2 wires with a bit of spare wire turn ignition on see if brake lights are on? switch maybe dodgey under load?