18 gauge wire from battery

You could also tap into the fusebox. The yellow (constant power) wire is also the main power for the radios internal amplifier, so it can draw 8-9 amps of current. If you run it to the battery, it should be at least 14 gauge wire fused with a 10 amp fuse, within a few inches of the battery. The red (switched power) wire can be fused with a 2 amp fuse.