Ok, I had an alpine installed with the harness that was like $100, then found a part that best buy put on to be able to hook up my factory radio in the back, about 3 months or so my alpine is now playing then all of a sudden it will shut off, do you think its the alpine thats bad or maybe since they disconnected the harness with this other one that also installed the factory radio in the trunk.

 

It could be alot of things. I would start by looking for a loose connection and trying to reconnect the stereo in the back.