Leak in the shower is leaking water through the ceiling

SInce it is wet both under and behind the shower I would rule out the drain floor pan and drain system and think it most likely to be the pipe leaking from the mixing faucets to the shower head. Find studs directly behind shower faucet, n each side. Locate the center of the studs and put vertical lines up and down on the center line of the two studs closes to but outside the shower  faucet on each side. Approximate 6 inches or so  up and down for the height of the shower faucet (Valve, not head) Put Horizontal lines across between the two line at both the top and the bottom/ You now have a square.Using a Razor knife(Sometimes called a box knife, stanley 99a for example) cut into the lines of the square back and forth as deep as you can. Cut an x  acrossthe square corner to corner again as deep as you can . Lightly hit the center of the x until the drywall caves in . Cut the remaining drywall out for the square. Now while  the shower is running you will be able to see whee the leak is. Use pipe seal after disassembling the loose fiting or repair the leak iun what ever manner is necessary. Replace and repair the hole made in the drywall  with another piece of drywall, tape and joint cement. .