Warning about this circuit: The dimming takes place by slowly reducing the gate voltage of the NFET as the capacitor discharges through the 4.7megΩ. The dome light in most cars draws more than 1A, which means that as the dimming goes through the half-way point, the NFET will be dissipating more than 4W.
If not heatsinked, it will get quite hot, maybe even damage it. Since you are putting this in a car, bolt the NFET to something metallic in the car. You will need a "mounting kit" consisting of an insulating mica washer, an insulating bushing, a screw, special washer and nut.