Do you mean the holder is dim no matter where you put it, or the position of the lamp is dim no matter what you put in it. On the first it's probably that the contacts are weak and not holding the lamp properly or the ends that touch the board and lamp are verdigris. Try giving them a clean or scrape up with a screwdriver and squeezing them up to grip the lamp better.

If it's the second, then look for bad solder joints on the board where the connectors go on, this is a fairly common cause.

Aha, I see it all now! Better get out the test meter. If these are on a circuit with other lights, then trace back to the previous one in the circuit and check for voltage drop, continuity and earth between them. If these are on a circuit on their own, then trace back to the board and check first for connector problems or pin solder problems on the board, then for the correct earth and voltage at the board with the connector off, one final check of the loom to the lights from the connector in case of short to earth somewhere, or finally for a driver on the board for this lighting circuit.

Good job it's a bank holiday, think of all that spare time you have now to solve the fault.

Good luck there.

Some of them, especially on the earth side are wired as a loop, so the ones after the fault can be ok still. It might be worth removing all the lamps except one of the duff ones and seeing if it's ok then. If so, check one of the working lamps hasn't shorted itself robbing the current from the others.

Looks like you're seeing typical ghosting on that LED. It's normal, but doesn't explain why it isn't coming on like all the others. Do those LEDs work in game?

Kind of hard to tell in the video, maybe the dim light I was seeing is a reflection.