For anyone struggling with this as I did, after many years, I have finally found a correct bulb type for the J-gate backlight, pretty much every single X300 that I ever driven had this bulb burnt out. Its a small thing but the car looks so much better with it working. Its a Lucas 286 type.

The blue cap is not actually needed, I've removed it in the end, I've used a grain of wheat bulb with a blue cap in the past but they aren't correct for this and just don't fit well and stop working fairly quickly. The 286 is a tight and correct fit.
Ski slope does need to come out, then you push the holder down and out and fit the bulb, test(super important before you put everything together) and get the bulb with the holder back in its place and put everything back together, 20 minutes job and great results!

 

This is pretty much regular T5 bulb size. They can be found blue in a color. But anyway all three different blue painted i've found or blu cap over clear will bring different color than oem tealish blue. And original looks brighter than 1.2W. Although burned again like in two months lol
BTW the bulb in the S-N switch was bigger issue to find a replacement. Best fit is T3 neo wedge with plastic base removed. Door controls bulbs look same too.