This is adapted from the HDX Wiring Diagram. I don't have a pinout diagram for the X25, but everything I see seems to indicate it's the same as the T8/T25, so this diagram assumes that to be the case.
This is for the PM 1200, part number 11961, without the expansion module. It will be slightly different if you have part number 11960-EXP, with the expansion module.
The PM1200 does not have aux audio inputs, so I have alert audio from the HDX routed to the music input. Be aware, this will cause your alert audio to mute when radio transmissions are being received or when the intercom is being used. That's a built in feature on the 1200 that can't be changed. Alerts will still be displayed on the HDX, regardless of muting.
There is a 10 ohm resistor added between the HDX and the 1200, to avoid loading and possibly damaging the HDX, as per the 1200 installation manual.
This is only an interconnect diagram between these boxes. It does not include other connections, such as power, PTT switches and headphone jacks. You'll find those in the respective installation manuals.
I make no guarantees, and invite others to correct any mistakes I might have made.