I just joined the forum having had real problem replacing a broken shifter cable on a Shimano Tiagra shifter. After following the post in this forum, cant find it now, I realise there are others who had same problem. My solution was grab the little bit at the broken end if it can be teased out the w top of the shifter, apply pressure and keep upshifting as if going for smallest cog. Sounds simpler than it is but anyway resist the temptation to pusth the big downshift lever (on my bike anyway) and persevere. Eventually the little knob will appear and the new cable can be fed stright in. For all I know everyone knows this but hope it helps someone out of a jam.