Just thought you might be interested in another, non destructive, way to remove the after market knob. I moved my F90BT to my new car, and didn't like the way the way the shiny silver knob looked. Since I now have steering wheel controls, I thought I might try just the factory knob for a while. If that doesn't work out, I may paint the silver knob. Instead of ordering a new factory knob, I put the original with the Hohl knob attached in the oven set at 200 degrees for 15 minutes. Be sure to remove the knob from the radio before trying this . After 15 minutes, hold the knob in a paper towel (it's HOT!) and use a pair of pliers to grasp the plastic knob shaft and pull straight out. It should not take much effort - mine just about fell out. The metal knob will expand with heat, but the plastic won't. This did not cause any damage for me. Now I can paint or order a different knob if I decide to.

 

Good luck!