I solved this problem years ago, the solution is a WURTH product called Rubber Care.
I have been selling detail products for years and only sell what works for me and my customers. Lots of "detail" web sites have their favorite stuff, but most often it has some baggage or some detailer pushing it. yes, I am pushing this stuff, but I don't care if you buy from me and I don't surf web sites pushing my business. I DO go here because of my history with the SHO and the Club and the conventions. Ask others here, I only speak up when I feel necessary.
Wurth Rubber care is a clear product that will take those hard pebbly plastic pieces that are discolored with wax (don't get me started on liquid waxes....! Stay with a quality carnauba paste). I found this out from a friend that sold the product years ago.
I sell it so do lots of others, but right now you can't get it in CA and some other states due to VOC's. It is safe to use on paint and plastics, and Wurth is working on a formula to meet CA's silly rules.
If you are interested, contact me off the list and I can talk you through how to use it and get good results. I tried Back to Black and don't like the result. Rubber Care leaves the item looking like new, but it may take several applications before the effect is permanent. It isn't expensive and goes a long way. Works on all this plastic like stuff around wipers, roof racks, caps on pickup truck bumpers etc. Also, is great for its main purpose, protecting and conditioning soft rubber seals. Great to use under hood for detailing on hoses and other black plastic.