If the emergency flasher works all 4 corners, obviously the flasher unit, fuse, and individual bulbs are OK. So, it's the switch.
This happened last week on my Ford van. Removed and disassembled the switch from the steering column, cleaned the contacts, reassembled, and all is well. This is a multifunction electro-mechanical switch with various springs, so be very observant as you disassemble it. Digital pics would be a good idea, so you can see how it all goes back together.
I dunno of Bentley has a wiring diagram or diagnostic for continuity checks for your BMW switch, but my Haynes manual for the Ford did, and was very useful. Tools included emory paper, needlenose pliers or tweezers, screwdriver and/or pocketknife, and continuity tester. Spray contact cleaner woulda been good too, from Radio Shack. My switch had green oxidation crud all over the place. so, I emoried the contacts, bent them slightly with needlenose pliers for stronger contact, lubed them with excess grease still in the unit from the factory, and reassembled. As always, something falls outa place and you have to figure out where it came from. That's why you wanna take pics going in. Trust me.
Dash01
'90 535ia 272K miles