Shower handle wont turn

This house had the shower pipe capped (no head). There are three handles on the tub diverter. Hot - Cold - Shower (center one). The former owner told the new owner they never used showers, just the tub. They lived there about 14 years. The hot and cold handles work fine, but that center shower one will not turn. I thought that would be a simple matter of removing it, applying a little plumbing grease and re-assembling it. It turns out the whole thing is plastic crap. under the handle is a metal outer sleeve that comes off, inside of that is a two part plastic stem, and below that inside the actual diverter is a plastic part which is the part that should turn (the cartridge). The faucet hot and cold cartridges just pull out. That center one will not pull out, and it's near impossible to get a good grip on the thing with a plyers or vice grip since it's plastic. I know too much force will break it.
Any idea how to remove the thing?
Also, are there any websites that show photos of tub faucets so I can determine the brand?
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