car audio, 05 galant, tail light fuse blows

There are a couple things that could cause this.  There is an illumination wire in the factory radio harness.  That wire appears to be a ground wire to a non experienced installer.  When the lights are turned on, that wire becomes a 12 volt wire.  If they connected the radio's ground wire to the car's GREEN / WHITE wire, you need to cut that connection and insulate the wire from the car.  Ground the radio to chassis of the car.

Second cause is a screw or a door sill clip has penetrated the insulation of the left parking light wire.  Go through every phase of the install and look for a problem.