Noticed that my wife had one tail light out when going down the road.
Bought spare bulbs
Took out the plastic carrier.
Which bulb has blown?
All look ok.
Get out trusty Avo.
All bulbs bus out OK.
The bottom spring connection on each socket seemed to be a hardened spigot which had caused indentation in the solder "pip" on bottom of bulb.
Switch on trusty soldering iron.
Touch up base of bulb.
All now OK. Spare bulb not required.
Conclusion : mechanical failure caused by erosion of solder "pip" by hardened spigot of socket.
Why?
Shouldn't be mechanical failure caused by vibration because of spring-loading.
But, perhaps after 5 years ..... ?