couple of things - anything on your dash needs to be dimmable - under ADRules. Therefore the best way to do this is wire the lights into the dimmer switch that is installed for compliance. Just make sure you splice AFTER the resistor - from memory the dimming is actuall on the ground side of most of the accessories. If you know what I mean.
Also, at the risk of stating the obvious, you need to use an accessories line (comes on when the key is turned on), not a 12v+ battery power line, lest your light will be on all the time and run your car battery flat.
also be aware that you need to be able to see the factory gauges and that anything impinging this veiw is yellowable.