Tell your dealer to do a little internet research and read the (I believe) Magnusson-Morrisey Warranty Act. A Federal law regarding vehicle warranties.
From what I 've read on it, the the manufacturer has to be able to show that the modification caused the failure. If you added 50 lights and burned up the switch from running too much current through it, that sort of thing.
What kind of additional lighting did you add? I added LED's to my trunk and saddlebags. But their current draw, like most LED's, is negligible. You have to add over 130 to get 1 amp of current draw.
From the parts fiche, it doesn't look like the turn signal switch is available separately.
Didn't say how old the bike is, but you may want to try spraying some TV tuner contact cleaner in the switch. It may just be gunked up.
Or take the lights off( if possible) and try a different dealer.
If it was me, in any case, I would be patronizing a different dealer from then on out.