We used to use something like this
http://www.pinballlife.com/index.php?p=product&id=1542
which involved connecting the 2 ends as Forceflow said to either the any bulb on the GI, or a featured lamp. If you connect to a featured lamp the light will only be on when that light is on.
Then the other end you would have to screw on to the underside of the playfield (after removing the light reflector of course).
But, also stated above, it's much easier these days with LEDs that you can put anywhere because they're so much smaller, and easier.