Casablanca Car

ok so i put in a 200 watt rms subwoofer that i found lying around the house in my car. Its a home theater sub so i bought a 400 watt continuous power inverter with a digital gauge, which i than hooked up to the battery. So the guage only reads that the sub is only drawing anywhere from 80-130 watts even though its turned up full gain. However, i only found a cable that has two rca's which convert into one even though the deck and sub both have dual rca's(red and white). So should i get a new cable since from the deck the red and white are converting into one and than into one of the subs red or white? Why am i not getting the subs full potential?
it has a built in Amp, but my 8 inch 165 watt sub sometimes uts out 200 watts, maybe the 10 inch is more efficient