Console disassembly 1984 400i

You should really check the ground of your front lights (with an ohmmeter you can validate there is continuity between the bulb holder and the ground of the fog-lights). From then on disconnect, clean, and apply "contact cleaner" on the followings: bulb socket, spade connectors, bolt and nuts of the ground. As a last resort you can make new grounds between the turn signal and the ground of your fog-lights.

As said, the ground is bolted on a location exposed to dirt from the road, you may consider a new cable here as this is not so much work.

You may also check the blinker relay, it is located on the left of the steering wheel, below the speedometer. I doubt this is your problem though.

 

As far as bulb holder and connector oxidation is concerned, you can soak these in salt & vinegar for a few minutes so as to eat the rust then soak them in water and baking soda in order to stop the acid. You can use more aggressive combinations (phosphoric acid / Ammonia & alcohol) for nuts and bolts.

Once the bulb holder is clean, I use lots of dielectric grease (baby petrol jelly) between the bulb and the socket so as to prevent the water from coming in again.