Shower floods behind the wall. on pre-formed shower/bath and ONLY when the shower is turned on
The easiest and first thing to check is the trim pieces around your valve knob(s). These trim pieces usually come with a cheap foam gasket, which crumbles apart after a while. Then water runs down the wall, gets behind the trim, and goes into the wall. 

When you take the trim pieces off, if the gaskets look bad, scrape them off and replace them with a bead of plumber's putty, which works much better than foam gaskets and lasts forever. Form the bead all the way around the trim, but leave a little opening at the bottom of the trim; that way, if some water does get behind the trim somehow, it will have a chance to escape, rather than accumulate behind the trim.