shock sensor on 2004 toyota tacoma rs3200

I don't have a truck sensor, in my case it's a tail gate and it is not powered nor monitored by the factory alarm.  I would love to splice it into the glass break sensor circuit and leave the door sensors alone.  I have no idea on how to splice it into the glass break sensor.  I know that I can adjust the sensitivity of the glass break sensor at the RS3200 unit.  Would I be creating more headaches for myself using the glass break sensor?  I thought the glass break sensor was a microphone and I really have no idea how to join the ground that the shock sensor produces. 

I read here on this forum, people recommend using a relay to power the shock sensor only when the vehicle is off.  Not sure of the pros and cons of doing it this way, just figured that this is the way it's done. 

Thanks for your input!

The glass break sensor (GBS) is hooked up directly to the alarm and uses a shield wire.  The GBS wires are colored black and white.  The glass break sensor inputs to the alarm, shown on the diagrams in the above link, have a symbol of MI+ and MI- .  I have a feeling that the RS3200 system uses the mic input directly and so the glass break sensor will not inferface with the shock sensor.  If that's the case then I have to use one of the door switches.  If you have a sec, can you look at the link above to the RS3200 diagram and give me your opinion as to which sensor I should splice into?  The RS3200 system is a factory alarm and power door lock controller in one as far as I know.   I am a little confused as to which switch to splice into because I'm not sure which switches the alarm is monitoring.  Thanks again!