Automatic dimming

[Damien]

In my car, there is a variable resistor that allows me to change
back-lighting level on the controls (e.g the speedo, radio backlight,
etc).

The purpose of this is so that the amount of light coming from the
controls should be approximately equal to the amount of light outside
(to reduce eye-strain).

What I'm thinking though is that I'd like this to happen
automatically. Has anybody worked on similar projects in the past?
Ideas of where to start?

Some areas where I need to think are:
Should I design the circuit to replicate the variable resistance, or
should I aim to produce similar voltage levels (how would I produce a
variable resistance as output of my circuit?)

How should I measure the internal illuminatin level - should I work
from the voltage I'm supplying (which would then lead to a need to
calibrate against real light levels) or should I reproduce whatever
light-sensor I have at the windscreen (so that I'm comparing two
identical sensor inputs)?

I haven't measured the actual resistance/voltages/current in this
circuit yet. I'd prefer to get a vague idea of what I'm building
before I start measuring.

Thanks for any help,

Damien