one possible solution is a special powersupply that is smaller sized than standard ones. i know a few ppl woh own some. these thingies are about 12x7x10 cm and can give 125watts which should be suiting. you could also simply use a standard 12v power supply, but it should be able to give more than 5 amps.shit is i don´t know the exact power consumption of a cigarette lighter.
if you haven´t got any spare room inside the case, why not mount the supply outside