interior dome light fade away, how to

There is no "simple" way to do this.  There was a thread on this a long time ago and I drew up a circuit but the person building it couldn't get it to work.

You basically need either the voltage or the current to slowly drop off once the signal is gone.  My circuit used a capacitor and a couple transistors (if I remember right).  Once power was removed the capacitor would slowly discharge through a bleed resistor (the relationship between capacitor / resistor dicated the fade time).  This created a falling DC voltage that was ran through the two transistors to power the lights.  You can't hook the lights right to the cap or you will drain the cap immediately.  The transistors basically isolated the decaying DC voltage from the load of the circuit (light).  I think, in theory, this would work, but i never actually built it myself.