I know a few of you need to light up a bike speedo, i've found this on the Halfrauds website, not to everyones taste but £4.50 and does the job:
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I use one of these on my Trike.
The light needs to come on with the sidelights for MOT purposes. I replaced the light for a high intensity LED and wired it for 12V fed from the sidelights.

I would be repulsed to find that in my car - is it SVA friendly though?