I have grave doubts that this is something that can be resolved quickly.
First though, are both left speakers silent? Adjust the front/rear fade and verify that the right channel works both front and rear. Then with the rear clearly audible on the right, adjust the balance to see if the left rear works.
If there’s no sound from the left, front or rear, then there’s a problem with the audio unit in the dash. Fixing that requires removing said unit and sending it out for repair, or replacing the unit.
If there is sound from the left rear, it’s still more likely that the problem is in the main unit, but by adjusting the fader to the rear only, you can have stereo sound for your trip – problem solved, for now. While it’s theoretically possible that the failure is in the left front speaker, it’s rather unlikely. The speaker is well protected from any possible damage, and speakers seldom if ever just up and die.
It’s conceivable that the wire to the L.F. speaker is broken where it goes through the door hinge area. Accessing and fixing that is somewhat of a pain, and I wouldn’t advise pursuing it to someone who is not experienced in that particular type of repair. ETA: Also, in my experience it’s power window wires that break first in this area. I don’t recall ever having to fix a speaker wire through the hinge.
The left front speaker can be accessed by removing the inside door panel (which again can be a challenge for someone not experienced), but would only be productive if the speaker has crapped out or its wire has come off, both of which are extremely unlikely.
Walkman type CD player and headphones?