depends on the type of lighting used in the dash. if they are LEDs, you need a controller that senses the state of the headlamps, and then runs through an iterative loop that gradually increases the pulse width to the LEDs before eventually settling on 100% duty cycle (full on).
if the bulbs are incandescent, i believe you merely need to vary the power, which can be done with the same type of controller, except instead of PWM as your power modulation, you may want to use a voltage divider network.
basic stamps are really easy to learn and implement for this type of thing. I believe all of the dash lighting in most cars originates at one point before being distributed, so I'd look for that point as well.
