Build your own amplifier based on TI's hybrid digital technology for the greatest efficiency.
Amanda Harris: the Prius of home audio - EcoRenovator
Just take that design and replace the power supply with one designed for 12v input. (Unless you're using it in an EV with a pack voltage range within 100-370v, in which case the design can be used as-is. The power supply will need to be derated at voltages below 170v but that should not be an issue as it is oversized already.) You can also eliminate the relay and its control circuit to save costs and Vcc can go as high as 30v for more power. Double up on the chipsets and drive the speakers individually. (The phase splitters can be shared for stereo, but they don't cost much and using separate ones will make your amp ready for surround.)
BTW, it doesn't take much power to get plenty of sound. I once used a homemade boombox (also based on TI hybrid digital, but only one chipset) with a total power rating of 20w and it worked just fine playing music in the car. (At some point, I'll install a USB audio input in my car radio so I don't have to use a boombox.)