There are usually two fuse boxes in a car now. One under the hood for higher amperage such as wipers, lights, brake lights etc. There is also one usually at the drivers door post. You can get a volt meter and test the fuses to see which do not have power while switched off. Make a note of them. Start the car and see which come up to 13 ~ 14 volts. You can get a "Add A TAP" fuse holder at the local auto parts an install for a power source.