My 2011 plus 4 only had one horn fitted. I fitted a second one about a year ago. Check if the one fitted says Hi or Lo, and buy which ever one is missing. I bought mine from Holdens. This is an inexpensive improvement.
Ian 2011 Plus 4 Wild Mogs Reims 2019, Goodwood 2018, Isle of Man 2017, Carried on to Caramulo" 2016