I've had the same problem recently.
There was a guy on Facebook selling the led boards separately but he also sold out of them all. I needed mine working for it to pass the MOT so I went to Toolstation and bought an LED strip. This one
https://www.toolstation.com/led-ip65-flexible-strip-light/p43662
Cut the wiring on the led down added some spade connectors on and voila. Unfortunately because of how I mounted the led strip inside, the leds are really noticeable now. I'm currently working on another one where I'm putting the leds behind the clear bit in the brake light housing for it to look a bit more Oem.
I might make a guide once I've done to help anyone else who may have the same issue.
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