<the red wire is live the black is earth.>

If, as T120 said.....

"there is a red and black wire coming from the headlight the black goes to the left indicator and red to the right indicator"

...then it would appear the left indicator circuit could have been wired using a black feed wire and a red feed for the right hand circuit?

As the bike is apparently a 1971(given in another post) T120 it probably wasn't fitted with indicators and the correct colour-coded indicator wiring originally?

I would advise caution before describing either of those wires as "earth". In any case, a T120 would normally be positive earth, so red would be "earth", and not black.

Also before around 1973 the wiring harness may not have had a red positive (earth/return) wire from the tail lamp? So the return current has to pass thrugh the tail lamp bracket, mudguard, frame etc in order to get back to the battery.