GPS or NAV switch for addon military aircraft
Is there a way to switch navigation to GPS or NAV from the keyboard. I don't mean SHIFT+3 to bring up the GPS; rather, instead of flying autopilot in NAV mode, which is the default for 3rd. party addons, I would prefer to fly autopilot in GPS mode. There never appears to be a switch in the cockpit to do this like there is in the stock FSX aircraft. Flying both the FA18 Hanimod and the F15J. Any help would be appreciated. 
