Installing a cigarettle lighter by my fuse box (under dash) in a jeep grand cherokee 05?

I bought a garmin gps nav it comes with a plug in for cig lighter, but I dont want the cords running all over the front of the car, so thought it might be easy to instal a cig lighter under the dash and run the wiring in there, and simply plug it in that way. I'd like to have constant power (ill be able to turn off the unit or unplug at the back of the device, that way the battery of the garmin will always be charged if i need it. is there an easy way to find out just what wire to use? i know the black goes to a screw to the metal of the car as a ground, i bought a circuit tester and have poked a red wire and the light goes on, but then as a test i picked some random green wire and did the same, the light went on too... how will i know if the power is constant, or just working if the car is running? I ume this isnt that difficult to do, but want to make sure i do it right and dont screw up anything with the car, or the garmin unit. any suggestions welcome, im sitting here waiting for an answer ready to do this! LOL.. thanks for any input and Have a Merry Christmas!