break contact relay

Silly questions but thought I'd check with you guys. Other than using a starter kill relay is there a different relay wiring or suggested relay that will break a wire when the relay receives a negative pulse. I am using this to break the door lock unlock motor wire when the unlock switch is pressed. I have the motor wires relayed to produce a negative output to arm and disarm my Compustar systems with the Ford door keypad. I then have a relay to break the unlock motor wire so that the alarm will not see the disarm signal if the door unlock button is pressed. This is the relay setup I'm using. Works fine but I hate having to run a 12 volt wire to it. Was wondering if there is another way to do it without having to supply power to the relay.

The relay must have Power and ground to allow it to energize. 

You might look into using a diode for isolation of the signals instead of a relay-