seeing as its both bulbs that have gone, i would guess that its power "to" the bulb that is failing.
the earthing point that you need to clean, never really gets anything wrong with it as it lives behind the gearbox side of the car's rear light.. away from water or dirt.
i suggest, checking the brake light switch and the power going to it (a simple ignition live)
if you have a multi meter or even a bulb on 2 bits of wire, you can check this quite easy, connect one lead to a ground like the cigarette lighter outer case or the radio's outer case, and touch the other lead on one of the wires going into the switch.
if the meter reads voltage or the bulb lights up, that would be the input wire to the switch. (one of the wires should make the bulb light or regester voltage on a tester if the ignition is turned on, if not it would be a fault higher up. more easy to fix by wiring in a completely new ignition live say from the light switch.
if it does not register voltage / light up, press the brake pedal and it should light if the other wire makes the bulb light/register voltage.
if still nothing but you do get voltage on one side, the switch would need replacing.
if you do not have a multi meter or a bulb on 2 bits of wire you can test it this way:
take the 2 female spade connectors from off the brake light switch and connecting them together. once you have check to see if the brake lights work.
if not, take a live from somewhere else, and connect it on to one of the connectors.. then check the lights.. if nothing connect it to the other and check lights again.
if the light do light up then the switch would be at fault.
it is worth doing, just so you know.
