Hi everyone, as a way of an update and sense check
Got the multimeter on it and was getting 0v and no continuity at the third brake light connector, so worked back and the connector directly before it was getting a solid 12v as it should.
Here’s where I could do with some sense checking…
The connector that the third brake light in my car was connected into seems (to me) to be a circuit to feed an unused and unneeded (for me) accessory plug to feed the boot rack brake light - like seen in this thread.
The third brake light was connected to this circuit, so I’ve unplugged it and put it directly into the previous connector that’s getting voltage. And huzzah, the brake light is working as it should again.
Above: the original set up, the conenctor the third brake light was going into on the bottom right.
Above: what I’ve done, connecting the third brake light directly into the previous connection and essentially disconnecting the accessory plug circuit.
To me, it seems there’s a short/break in the wiring to the accessory plug and all I’ve done is re-establish the circuit as it should have been originally - disconnecting the luggage rack wiring harness essentially.