3M Adhesive Remover
Try that, put it on a clean cotton cloth, NOT a terry towel, and rub. Be careful not to rub too hard, and have soap and water on hand to rinse/neutralize it. This was the only product I've found that will remove car wax from black plastic trim. I also use 3M's blue masking tape to mask off door handles, etc. when I wax my truck. The adhesive remover also works wonders for getting bugs/road tar off the paint, but strips the wax right along with it. (no problem, it needs a wash & wax by then, anyway)
Some automotive body shops have wax remover in 1 gallon cans, you might have to try a few shops before you find one willing to help. I think both 3M and DuPont make such a wax remover, I can't remember for sure. The danger is some compounds can act as a solvent on the plastic itself. If in doubt, find some scrap car plastic at a salvage yard, and test it.