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      )