Light bar turn on when ignition is off

I am wanting to install an alarm in my daughters car but temporarily I want to use a light bar and have it so it has the 12v constant always connected and when the car ignition is off it will provide a ground connection to the light bar causing it to activate. How would I wire a relay to do this? I am wanting to do it this way so I can easily wire it into the wiring of the alarm when I arm it when I get the alarm later. Thank you all in advance for your help. :)

POS side to a 12V+ constant feed, NEG side to 12V+ IGNITION. The NEG side is then transferred to a GWA (ground whilst armed wire) on installation of the alarm.

Thank you for your reply. I dont have an alarm for this yet so I am setting this up for now so that it turns the bar/scanner on when the car is turned off. I hooked the led scanner up to 12v constant then tested direct to ground and works like a charm. Then I connected the ground connection for the scanner direct to the ignition wire instead of ground and it turns the led scanner on with car off but doesn't scan as if it isn't getting enough power or something. How else could I wire this?

Try ignition wire at the ignition switch.

thank you. I decided to go with a relay and it works great. I used the ignition wire as a trigger for a standard Bosch relay that rests at ground when the ignition is off.   Pin 85 to ground. Pin 87a to ground. Pin 86 to ignition. And pin 30 is supplying ground to the LED Light/Scanner bar.

Fair enough