110 Bad earth - where to look

This sounds as if it will be an earth point somewhere in the left rear near the fuel gauge sender, earthing both the sender and the brake and tail lights. Rather than look for the actual fault, you could try an extra earth from the earth terminal on the sender to the chassis nearby - a temporary one will confirm the diagnosis, but a fix could be to make this permanent. the sender is accessible between the chassis rail and the body through the LH rear wheel arch - easier with the wheel off, but possible with it in place. JD