The scanner keyboard will have contacts under the buttons, usually bridged by conductive surface on the underside of the keypad. If you aren't concerned about the appearence of the scanner, the keypad can be left off. I know someone who did excatly this hack 15years ago, back then he used the parallel port to control the switches. Given the scarcity of parallel ports these days, a PIC solution may be the easier. I think I also have a Pro 58, the only mod I made was to extend ??? (can't remember) to the outside so I could use the signal to drive a homemade deviation meter.