As the saying goes ....been there, done that.
I've adjusted prongs solved it once, next time it was a bulb that looked OK but failed on continuity check. There may be weak contacts in turn switch on these vehicles 'cause sometimes I can get 2x flash when activating lane change feature lightly, pressing harder seems to solve it.
I've no specific suggestions, just thought I would aknowledge your post before going to lunch.
Edit @ 4:00pm. Just so you're clear on operation there is no relay, only one flasher, and many switch contacts involved in brake/turn/hazard circuit. Each corner of car has its own contact for turn operation, therefore you've isolated problem to that contact(RR), wiring to bulb (including prongs) or bulb itself. I'd bet on prongs or bulb first, then wire connection at bulbholder before going further afield. BTW RR brake light comes from a different contact in turn lever, but via the same path externally.
