If the high-mount 3rd brake light still lights up, then the problem is the turn signal/multifunction switch, as is not wired through the M/F switch (but all the power for the left and right tail/lamp turn signal assemblies is supplied through the M/F switch). If even the 3rd brake light is inop, then the brake light switch on the pedal is most likely the culprit since all the other light functions work.