Advice, big problems

I'm 99% sure that the problem is my factory amp. I have the stock cd player and ran the rcas off a line converter connected to my front door speaker, it was just powering a sub and at most the amp put out 800 watts. I had no idea what i was doing at the time and used 4 guage wire instead of the recommended 8 guage. Dont know what happened exactly most likely the power and ground touched because the wire was too close but when i turned it on the system shorted. The strange thing is only my front door speakrs work now, not the rear door speakers or the speakers in the back. I took it to music in motion and they told my my factory up had blown channels. I would take their word for it but it seems like they were trying to take my money because they said in order to fix it i would need to purchase from them  several things that i didnt need, such as a new amp and new speakers. The speakers work fine there is just no signal getting to them. I tried using the rcas into a less powerful amp and running the speakers directly off of them and that works except when the car is on i get a whining noise with the rpms. I dont think it is because of my rcas being next to the power wires because  my  battery is in the trunk, where the amp is. I have heard that buying a new cd player would fix the whole problem, but i dont want to do that unless i need to, the car won't be mine for more than a year. I thought that maybe i get the whining noise because i am using a low leval line conerter to get the audio signal. I only get the whining noise when the high pass or full filter is on. Any help is appreciated and if you need more information just ask.

The reason the 4 guage wire was a problem is because it didnt fit correctly , and i think it shorted because the ground touched the power wire. Not completely sure thats what happened becaue i wasnt looking in the trunk when it happened. I know that blown channels in the amp is a major possibility, and when i said amp i meant the amp i was using to power the subs not the factory amp. Also, i hooked up the speakers to a different  and they worked fine, but i have new speakers in the rear doors anyway. Thanks for the tip on power wires, its obvious but i never thought about it, i know for sure that there is power carrying wires where im picking up the audio signal from.

I didn't know Delco used anything with that low impedence- wow.  Yes, that's definitely a heavy load for any amplifier.