Jack,
Assuming no one has made after market changes to your wiring, you can check the fuse/breaker box to determine the potential load on each wiring circuit. Bigger fuses with higher amperage rating means those circuits are designed for a bigger electrical load. Cigarette lighters normally pull lots of power, so 3 amps or about 40 watts shouldn't be a problem. However, electrical mods may be something you would feel more comfortable having someone experienced complete, or at least check your work. An error can have serious consequences.