I’m not certain but given its location and the number of terminals and wire colors, I’m going to guess it was an added horn relay for dual horns.
Check the download manuals tab on the left. Then select Classic Motor Carriages, part 2, page 40. The manual shows a horn relay with three connections (Red, Brown, and Violet.) You can see in your photo the red and brown wire and a vacant terminal that would have had a violet wire (unless the builder used a different color.)
Just a possibility!
newkitman2014-09-17 12:58:34
Allen Caron
VW based 53MGTD - "MoneyPenny"
"If one thing matters, everything matters" - from the book The Shack