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I can't think of anybody who makes something you could buy off the shelf that would work for this.
You could certainly build your own circuit to do it if you had some electronic skills. A low voltage dc motor control circuit would probably work. They have hobby circuits like this for robotics. You can do a search on the web and probably find a schematic that would work.
It's a long shot, but you may also be able to go to Napa and find a universal heater fan speed control. It's a fairly large variable resistor, that would probably handle the load of the lights. My old 53 Ford had one added in, but I don't know where they got it from.
Radio Shack can fix up up with a variable reistor. A potentiometer, or pot. Just measure the current draw so they get it right the first time.
I doubt they have one big enough. It will need to be fairly large, and be the wire wound type with the ceramic form.
I don't know what the dome light bulbs draw, but if it was ,5 amps, the worst case for the resistor would be 12 volts x .5 amps = 6 watts. 1 amp would be 12 watts. Most of the pots they have a radio shack are barely 1 watt types.
That is a switch and a potentiometer all in one. The switch is rated at 3 amps. They do not give a rating for the resistance part of the unit. But they call it a "mini style" which means it's probably even smaller than a 1 watt type. And 10,000 ohms is way too much resistance for this application.
You can try it and see what happens, but I bet you get smoke out of that one.
i think hes refering to how newer cars dim off when you shut the door, instead of just turning off? am i on the right track here? you might be better off finding a car at a junk yard with the circut already and pullin it and splicing it in.