Most car clocks will also have a third wire, that is used to dim the light when the healights are on if its digital, or to turn on a light if its analog.

You may or may not need to have iot hooked up. Its usually also a 12v feed, so you can just twitch it to the positive for the clock unit.

But yeah, one to earth the other to 12v positive (assuming its a 12v unit.)