There is a kind of screw that was used by GE to screw in to plastic. They were used inside of Mastr II and other radios, to hold speaker cases together, and to hold plastic cased control heads together. A selection of them was also included for use in mobile installs with each mobile radio purchased. These screws had two sets of threads (like a double helix), one thread taller than the other. I have not found anything that works better at screwing in to and holding to plastic, in vehicles and other applications. I am pretty sure that "Plastic Tapping Screws" is not the correct name for them. That is just what I call them, for lack of a better name. I'd like to know what they are really supposed to be called, and where I might be able to purchase them.