OEM Flasher case does not need to be grounded.
Interesting ... I just looked at the FSM wiring diagrams and sure enough ... no grounds.
Just assumed 'cause some aftermarket units have ground pigtails that the originals were grounded.
I do not recall ever seeing a flasher that needed a ground with incandescent lighting. LED lighting may be a completely different story.
even the somewhat later model K cars did not use a ground wire on the flasher can[s].
however, when the flasher cans were combined into one [square] unit in [or around] the 96ish [?] model year, using a multiple prong unit, i don't know if those are grounded or not.
i never traced it to see.