Jeresous

So about a year ago I had a ton of car stereo work done on my car. I did none of it, because I'm clueless when it comes to making that stuff actually look good. Almost immediately after my new front passenger speaker began cutting out. Not for brief amounts of time, but for hours or days. I had the guy that did the work take a look at it and he disassembled everything and followed all the wires to make sure all connections were solid. They were. I then wound up changing my car battery later because the previous one had been having issues since before the stereo work. After the replacement the speaker stayed on just fine for many months. Suddenly it has cut out again... coming back on for a few hours once or twice. The last time it came on was weeks ago though so I think it might be out for good this time. The best I can think is there's some kind of voltage problem? I actually had the car battery fully charged today, and it's still not coming on. Any ideas?