Haven't fitted it yet, but I'm also thinking along the lines of teeing into the existing feed using Superseal connectors and then attaching an LED aftermarket brake light to the luggage. The one I have bought is IP68 (high standard of ingress protection for electronics), has an adhesive strip which should attach to the luggage strap, and is rotatable so that the light is correctly oriented.
I do wonder whether the adhesive is man enough for the job, but that can be rectified.
high level brake light
