[FrankM_]

Careful with that, the 'solid red wire' is unfused from the battery over the contact switch to the light switch ! And is 'ALWAYS HOT'
It should have a good solid contact on terminal 30 of the light switch (clean contacts), no 'jiggling' required.
The terminal 56 of the light switch provides power to fuse 3&4 (fusebox II) and runs the headlights.

Most plausible : check your fuses, left and right side are separately fused and separate fuses for normal and high/beam. It is fuse 1 to fuse 4 on the fusebox 2, 4 fuses of 8A in a row.
Fuse 3 runs the left headlight - blown fuse or bad fuse contact ?

If not that and you have replaced the bulbs : a wiring issue ? Do you have relays installed for the headlights and high beams (non standard but highly recommended) ?