This is the supplier and part, aftermarket driver for led conversion
https://www.powerfuluk.com/collections/l322-06-lamps-lights/products/led-rear-light-wiring-fix-kit-for-range-rover-l322-02-09-cars
Hello and welcome to LZ :)
Best to ask tech questions over in the disco section of the forum https://www.landyzone.co.uk/forum/land-rover-discovery.8/
have you checked the battery voltage as low volts will throw a fault up on start up.