SoCal, with a semi-severe lack of trained slingshot mechanics.....is it possible that the person you talked to doesn’t know about the flasher unit Sam is talking about?
You have the small flasher relay and then you have the main flasher unit under the dash.
The guy you talked to does not understand the Turn Signal System. That relay is really miss labeled. It really is the Secondary lights including the side marker lights. When it turns on it just powers up all those lights. You need a relay because it is too much current for just wiring. That is what a relay does, takes low current circuit and operates with the contacts proving high current drive. The relay does not have any part in the Flashing of the lights. There is only one flasher and it is underneath the Fuse Box and ECU. Now don't get scared, it is not hard to get to, just watch my video. I do these videos to help with folks that have never tried something. As an instructor in the Telephone Company, I know what folks need to understand how to do something. My video are boring to folks that know this stuff. I did not make it for them.
Let me make this clear. I am proving information from my knowledge of a 2017 Slingshot. I don't have a service manual for 2015, I know they did add the secondary light relay after some problem with the 2015, all lights were power through the Primary Light relay and that caused problems as people started adding lights system. We are talking just about the Flasher Module though, forget the relays.