S7 gauge question
 

I bought one of those hidden face AC autotechnic S7 gauges and it came with three wires. The black for the ground, The red for !2v when you turn the lights on and a purple wire. Now my problem is with the damn purple wire. It says that when you turn on the ignition and start the car it will light up. I guess the purple wire is to let you see it during the day. What in the world am i supossed to hook this wire up to? I have the black grounded and the red on the wire for the light bulb in the ash tray of my civic so when i turn the lights on, it is on. THe wire is red and black. Any suggestions? The diagram was lost.