I've done this job twice now: the first in 2003/04 then again a few months ago. This most recent time, I ordered the spoiler/light and wiring harness from David Silver Spares almost a year ago, just before they notified everyone that they would be halting all accessory sales to bikes younger than 10 years.
Here are some pics from the final job:
When I did this same task in 2003/4 on my first (silver) ST1300, David Silver Spares wasn't selling any wiring harness - you had to make your own in those days. This is what that silver one looked like:
Thanks to one of the US forums, I was able to get detailed instructions for wiring. These included installation with diodes and resisters to make the spoiler light a
combined running and brake light. I have not yet done this job on my current ST but one day will (I have the parts ready to go).