Go to the junk yard, Im sure u will find what u need, and it will be alot cheaper. Sometimes the person working there can put it on, and since putting a side mirror on is very easy. He/She might do it for free.
The 4 gauge is connected to the battery. Do u have a little fuse box with a 80-100amp fuse connected to the wire. If u do check and see if u blew the fuse. If that's not it check to see if your ground wire is connected tightly to the amp and to a metal part of ur vehicle. If that isn't it check...