Yes, even looked it up. Weak spring in the return circuit.
BSA/Triumph/Norton etc typically had a brake light switch mounted to the swingarm, with an external spring, like most old cars/trucks, and dust/dirt gets into the switch or the spring gets mangled by a twig. Oddly enough here's a brand new one I have in a box, from where dunno. Maybe I'll get a bike to put it on must have been what I was thinking.
You can see the rubber boot that prevents ingestion of contaminants, when new.
The part in your car could be remarkably similar.
FWIW, while we continue the veer, I put nearly endless miles on BSA/Triumph/Norton/Matchless and a Triumph GT6 and have no particular complaint about Lucas electrics. Bulletproof in fact. Lots of old timers - I included - with BSA singles - 441 Victor - and ET (Energy Transfer) completely batteryless ignition could attest that it probably as fine an ignition as anyone has ever made.
The Matchless 600 single had a Lucas Mag, about as reliable as a rock.
The GT6 had no electrical problems, I would have driven it anywhere, and did.