Mystery Delta faucet leak in shower

I have an old 3 handle Delta setup in a fiberglass shower. The handles are clear plastic. When the shower head is pointed down and splashing the handles, water comes through the wall of the shower. When pointed away from handles there is no leak. I've sealed everything I can think of yet it still happens. I siliconed the chrome cover plates on all 3 handles, I unscrewed and sealed the chrome sleeves at both threads, also sealed the tub spout. The leak appears to be coming through the center valve area and is dripping through the fiberglass. There are no cracks in the fiberglass wall. The only place I haven't been able to seal is the valve stem itself. Is it possible that water is splashing into the plastic knobs, through the valve stems, then into the wall? If so how can I seal the stems, or can I buy new ones?

Your diagnosis may be faulty. It would be possible, but highly unlikely, that water could leak through the escutcheons. Use water in a soda pop bottle, and POUR it in the various locations. Wait a while between areas so the water has a chance to leak out before you move to the next point.

When you seal around the escutcheons you should leave a small gap on the bottom so that if a little water migrates in between them and the sleeves, it has a better place to go.

Great ideas, thanks for the help. Will update with results.

trim against fiberglass always leaks over time as the escusions loosen up from use and the fibergalss flexes from the temp changes. I always make the sure the valve is strapped really well, then try to putty the hole around the valve if possible. I do not use regular caulk, I use silicone and lay a bead down under the escusion and screw the escusion on squeezing it out. then taking extra care to clean up the trim and fiberglass. I have never seena Delta stem leak behind the wall, but have seen price pfister compression stems leak all the time when the packing nut gets loose or the packing gets worn out.