There are 2 ways of doing this.
1) Buy a stabilized power reducer. to run both your head unit and amp. This reduces the voltage from 110v to 13v.
This will be expensive for this output you require (300w amp)
2) Run the head unit and amp from a 12v battery. Then use a battery charger to keep the battery topped up.
The head unit and amp will draw more than the charger can put in, therefore the charger will need to be left on when the radio is off to recharge the battery.