Since buying my Plus 2 a couple of years ago, I have experienced various electrical gremlins , including bulb holder wiring. What’s common to all the external bulb holders is they comprise a spring, a mica insulating disc and a contact ‘mushroom’ attaching to the wire ( two in the case of the rear tail/brake light holder. What goes wrong: the wire breaks from the contact, the insulating disc breaks and the spring corrodes or ‘pings’ away.
The solution: buy bulb holders for Ba9s (front side lights, number plate lights), Ba15s (indicators) and Ba15d ( tail/ brake), easily found and about £3-4 a pair on ebay then carefully open up and remove the component parts to use as needed in the Lotus bulb holder. You may need to trim off the tiny ears on the insulating disc to get the even round shape needed for the Lotus bulb holders.
One final suggestion: reinforce the crimped join between wire and mushroom contact with solder and then adhesive heat shrink.
Tony