The high mount brake light uses a seperate switch from the stop lamps. The stop lamp switch sends voltage to the multifunction switch on one wire. The multifuncton switch divides it into two separate circuits that go to the rear. The 4 way switch on top of the steering column splits it again between front and rear. Just for grins, operate the 4 way switch a few times and see if that makes a difference. Could be that the contacts are dirty or the switch is partially on messing up the circuits.