I've looked into options for adding a usb plug/port in the cigarette lighter location on and off for well over a year.  A few days ago I got a break got idea to try and mount the tab plug into the lighter knob itself.  Of course I'll figure a way to modify the assembly to accept the plug and keep a stock looking lighter setup.  I'll mill a slot in the end of the knob and tunnel the wire through the middle of it all.  It won't be that straight forward but I'm pretty confident I can make this work.  May even be a market for such an idea.

The redline in the picture on the knob is where I want to mount the usb end. I've already bought a plug end and DC 12v to 5v.  I'll bench test it all before I put it in the mustang.  Just need time to start on it while I'm painting my truck project.  lol