Firstly, you need to check the LH bulbs are the correct type (12V 21W). Check the 'AMP' mini-plug handlebar wire connectors as they are barely adequate electrical conductors when in perfect condition (replace with bullet connectors).
Then, I suggest you check the earths/returns of both LH indicators as the Lucas indicators rely on the metallic coating of the plastic bulb housings for their 'earth' between the bulb holder and stem.
Also, the stems may have poor earth connections, the front indicators rely on the red wire connection inside the shell for their return (the headlamp shell connection is not a headlamp bulb earth/ground as often thought).
The rear indicators rely on continuity through the T160 grab rail and bolted connection to the frame for their earth/return, unlike most other electrical components which have a return wire.
As a check, remove the indicator lenses and run jumper wires from the bulb holders to a known good 'earth' (red wire or battery positive terminal, etc.) and see if that improves/cures the problem? If it does, an 'earth' wire can be added to each indicator by feeding it through the stem and connecting between the bulb holder and a good earth point. As it's an earth, a single strand of bare (copper) wire will do (photo);
If that doesn't do the trick or it does not completely cure the problem then the indicator part of the handlebar switch cluster can be dismantled and cleaned easily enough just make sure the brass contact and spring in the nylon slider don't make a bid for freedom!
(Actual switch cluster shown below is from a T140, your T160's cluster will probably have an additional blue/white wire going to the 'horn' button switch for headlamp flash)
