How can I repair my shower that is leaking from the back of wall?
Gotta open it up to find out.  You can always open the drywall on the backside to get a look into the mixer valve and pipes in that area.  Drywall is much easier to repair than tile and any fixes can usually be done from there as well.  Also you might want to consider adding an access door so inspections/fixes will be easier in the future.