Can find no cracks in fiberglass shower/tub combo. About 40 years old. Water comes out from under unit at wall. Handles leak water when turned ON, could this run back into pipes, down pipes to floor then out at wall at opposite end of shower.. need to isolate and fix it. Can't really see in drain. Not sure how to approach this since I can't get to anything other than take the handles off. The faucets do not drip or leak when shut off. HELP, SUGGESTINS, OPENIONS