I just picked up my 2017 LEAF SV a couple of weeks ago ... first thing I did was research how to replace the "meep meep" with something a little more suited to highway travel. As others have posted about the 2015+ models, getting at it from the bottom of the car was easy and straightforward. Here's some pics I took of the "El Grande" install.
New "Get outta my way" horns next to the OEM "meep meep".
Only takes 5 bolts and a 10mm socket to get access from the bottom of the car. Jack from one side and put a jack stand under for safety.
What the stock setup looks like. One 12mm bolt to remove it from the support.
The two connections to the stock horn. The longer cable with the black connector is the ground wire that connects to the bracket. The shorter yellow connector goes to the pigtail male connection.
The new horn all bolted up to the stock bracket, horns angled down and a single ground connector attached to the brackets.
That was easy! Like others have said, the pigtail cable you need to create only really needs to be 8 inches long at the most. There's plenty of wire on the stock harness. The whole job took like 30 minutes, mostly because I read through this thread before attempting. Thanks for the help!