The only common point in the indicator and horn circuits is ground G103. The location isn't shown in the FSM but the ground is also shared by the washer fluid level sensor which might help you locate it.
There's been a similar issue affect some TJs where a bad ground can cause the wipers to be activated by use of horn, headlamps or left turn signal.
Hopefully it's as simple as cleaning the ground up when you locate it. I can't explain why should continue after the turn signal is turn off though. I do find that bit puzzling.