The bulbs are just standard single filament 21w bulbs, the same as you find in a standard indicator or sidelight, very easy to find.
If you look at the pictures you will see the mounting threaded bar, this will unwind from the light fitting and I was going to replace it with a longer threaded bar so that it went straight through the fibreglass, that way you just undo the 2 mounting nuts and you can remove the base from inside the car to replace the bulbs. Fit with LED bulbs and the chances are you will never have to replace the bulbs.
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