The T5 is also used in the MK2 F hi-level brake light. 7 of them.
If you want to use leds in this position, you will need to enlarge the apertures in the reflector. As standard, the t5 led bulb won't fit.
I enlarged the apertures with a 6.8mm drill.(That's the size I had), but a 6.5-7.0 mm drill will do the job.