In the aircraft's main folder, there's a file named "aircraft.cfg"
For planes with nav-radios using standby frequencies,,, look for the [Radios] paragraph and make sure it reads exactly like this :
[Radios]
// Radio Type = availiable, standby frequency, has glide slope
Audio.1 = 1
Com.1 = 1, 1
Com.2 = 1, 1
Nav.1 = 1, 1, 1
Nav.2 = 1, 1, 0
Adf.1 = 1
Transponder.1 = 1
Marker.1 = 1
That should take care of it..
(make a backup first

)