If you have an old computer, you can use it to power a head unit.
Just pull the power supply out, find the 12V rail using a multimeter, or if you build enough computers you should just know. Find the power and ground wires, usually they are red and black, and cut them.
Run the power wire from the computer's power supply to both the power and remote wires on your head unit, and use the ground as a ground.
Then find the ATX cable on your power supply (the big one that plugs directly into your motherboard) and use a paperclip to jump whichever two pins cause the power supply to turn on.