I can see enough reflection in the glass that it confirms the brake lights are working (a nice unintended benefit – the quality of modern replacement brake light switches seems very inconsistent and some brands are hopeless), but it is not enough to be even slightly distracting, even at night. I didn’t make any effort at all to seal the light system against the glass, but being LED, is tends to be fairly directional and shines the vast bulk of the light outwards.

These are the people I got it from: s-v-c.co.uk