Todd list your vehicle and year in your signature. You will get MUCH better answers
Usually (at least in 80's models), the ground wire is usually dark brown (sometimes black). In any case, the ground wire will most likely NOT run back toward the fuse box. To feed the new wire, tie/attach/tape the new wire to the end of the old wire, and pull it through (ie, pulling the old on out pulls the new one in.). Good luck!