^ the 'power cube' is basically a three dimensional graph of voltage output versus impedance at differenct phase angles due to varying capacitance and inductance.... some company sells some device to measure all this...
Anyway, there are digital wattmeters that you can buy... but anything decent is going to be expensive, and isn't going to be ment to be installed in a car's dash
I have a feeling the OP is after some install candy, in which case I'd look into installing a VU meter.. you can get analog VU meters, or ones with LEDs that light up...
look on ebay