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Brake light switch behind your pedal. Check it

Check that the centre contact of the socket still gives +12V when it's depressed, as if a bulb was in it. Otherwise just try changing the bulb anyway. I've had perverse house light bulbs that work in one socket and refuse to work in another for no apparent reason.

BTW the wiring to the boot mounted stop light is a common failure point in NA8s. The copper inside the wire near the boot hinge loses continuity while looking perfectly OK inside its plastic casing. Sometimes it will work with the boot open but not closed.

I would change the wedge base bulb to eliminate it. If you have power to the connector with brake down then the fault must be in the bulbholder or the bulb. The other thing to change is the wedge base bulb holder- presumably you could get one without too much trouble or borrow one if you have a mate with same car.

Does that say you've got just enough power going to the lamp holder to enable a 12V reading on a multimeter but not enough to drive a bulb? I.e. the circuitry fails somewhere when it's drawing more than a couple of watts. It could be interesting to put a pot and a multimeter across the lamp holder.

You could be working at the wrong end of the car and the brake switch or the fuse or the fuse holder are still worth checking.

Edit: correction, just read about the other brake light working. It must be weakness somewhere in the centre light loom. I'd try temporarily bypassing it with jumpers to see what happens.

I reckon its the bulb holder. The b/holder is usually 2 wires with a terminal in the b/h. Either there is a break or a bad connection which does not pass current. I would substitute the b/holder with another.Some bublholders have a terminal pushed into them which locates in the moulding and held by a sprag on the terminal. The connection with the bulb is made with the terminals and the elements on the bulb- a visual examination might be in order as well.