Do you want to make this loud, or do you have a better reason? I've seen home stereos installed in cars, but always as a gimic. They are not made for the rough treatment. If you are familar with military shock requirements for electronic equipment, then you may realize what specs car stereo manufacturers use in designing and manufacturing those little boxes.
Most car alternators put out 140 amps with a voltage anywhere between 14 - 16 Volts DC. Most DC to AC converters for auto related junk are not regulated and therefore you AC voltage will vary between 100 - maybe as much as 140 VAC. This in not good for household electronics. You will not only require another adapter, but a requlated AC power supply.
