BMW never did one
Heated seats only came in on 1200RT
You may find someone who has done a homemade job - but in 25 years of oil heads I have never heard of heated seat for a 1100/1150 RT which use same seats
Sargent or Corbin may be options if anyone has a heated option for sale used, but as said I haven’t ever seen one
Unless homemade (heat pad insert from Wunderlich, either external, on top or internal, in a reupholstered seat) I think he's mistaken and confused with a 1200RT
Phil there is a live and and earth under the seat for a phone charger were to tool kit is at the back mate .
as said i am sure you could get a kit of ebay ,take the seat cover off fit it,and re staple the cover back on