These are a couple of links to circuit diagrams of the Yaesu FIF-232

Pin Out for DB25 and DB-9

THIS is link to FRG9600 Service manual

The DIN cable is extremely hard to find, so if you DO find one, I'd recommend you buy TWO,
especially if you intend to modify a cable for Squelch Detect

DIN Cable Pin 5 is used to indicate "squelch open/closed".
Yaesu in their infinite wisdom put it on Pin 5, but never went any further
in that development. So the rest is up to the user.

 The Voltage from this pin can be used to then trigger a pin on the serial cable.
Using a DB-25, Normally Pin 20 and 22 are not used, so you can then "tie in"
the pin 5 voltage to one of those pins - to then get a squelch open indication. 

The problem "is", the PIN 5 voltage is not RS232 compliant, so being it's
5.5 volts and the serial cable wants 12 +- , there is a problem unless you "step up"
that voltage so the RS232 can "see it".
You will have to contrive some form
of "voltage level converter" that creates 12 Volts using a resistor/transistor circuit. Otherwise it won't work.

THAT is as far as these instructions can take you, so have fun