Not quite sure exactly what you were refering to when you say "molly", probably me just being ignent. Whenever I need to mount anything solidly and not attach directly to studs then I use toggle bolts. These are all metal and the end of the bold has two metal wings that have a spring on them so that they can fold up along the bolt shaft and then after going through the drywall and/or cement board they spring open and give very good support. If possible, I would try to anchor one end of the bar on a stud. Usually tiled shower areas only have the tile go up so far, and there will be a regular stud and drywall wall above the tile. It should be easier to find the stud locations up there, without the tile in the way. Then, once you find one, you can just measure out from a corner and transfer the location down.