O9 Head VIP Inputs on 7500 V/U

[carcomm911]

O9 Head (on a APX7500 V/U) VIP Input Help. In CPS, R33.00, I have set the VIP INPUT functions as 1: Aux Siren Sw and 2: Horn Ring. Pinout of the VIP yellow plug on the back of the O9 head shows Pin 8 & 9 are the signal input pins. I have tried shorting the #2 GND pin to both 8 & 9 and get no results. I did notice in the many documents I reviewed pertaining to this, that the "Yellow accessory cable must be plugged in for features to function", or something to that effect. I have yet to receive my cable, so I am bench testing with jumpers. Further, in manuals I have read, I believe I am understanding that the VIP Inputs are in fact looking for a GND signal, and they "go through a buffer" internal to the radio before signal is processed.


Am I shorting the wrong pins?
Is there some sort of internal "sensor" that knows when the VIP cable is plugged in?
What am I missing here?
Thanks in advance for any input you can offer.