What is wrong with my stereo?

What's wrong with the car stereo in my 2003 Nissan Sentra?

• What's wrong with the car stereo in my 2003 Nissan Sentra? I have a Blaupunkt MP3000 dash unit which has served me well. I had it professionally installed shortly after purchasing the car (new-ish), and until recently I've had no problems with it. (No external amp or anything fancy like that—just the head unit.) A couple of months ago, the audio started cutting out intermittently. Going over a bump in the road will often cause it to cut back in briefly—and it worked for a short while after I removed the dash panel and disconnected and reconnected the wiring clips—but those periods of functionality have gotten shorter and shorter, to the point where I only hear an occasional split-second of radio when I cross a rough train track. It's not any one speaker that's cutting out; they're either all working, or (more often) all not working. The Blaupunkt is receiving power, and from all indications is performing normally—the channel meters show that an audio signal is present, and everything works the way it should. It doesn't matter whether the signal is coming from the radio, the CD player, or the aux input—it's just not getting to the speakers. I tried removing the Blaupunkt and putting the original factory unit back in—and got the exact same behavior. Unit gets power, seems to function normally—but no audio. Since it's getting power, it shouldn't be a fuse issue, but just for shits I checked all fuses that sounded relevant—under the hood, inside the cabin, and on the rear of the Blaupunkt—and all of them look fine. When it was still working for short periods of time, it seemed like it would cut out again when the signal got loud (e.g., if I turned it up moderately loud, or when the broadcast/CD peaked)—but I couldn't be certain of that. (If it matters, I replaced all four factory speakers with new Kickers a few months ago.) So what's the deal? I thought those eight wires coming from the back of the head unit connected directly to the four speakers. So how is it that two otherwise functional head units are failing to power all four speakers? The behavior with bumps in the road suggests a loose wire/component, but what single point of failure could possibly cause this behavior? Obviously I'm misunderstanding something here—do the four speaker signals pass through some common component inside my Nissan before they get to the speakers? Any suggestions would be welcome—I'm starting to go crazy without music in the car, and I can't really afford to take it to a professional right now, and besides I'd rather fix it myself, if I can.

• Answer:

  Was this one of those setups where the new head unit came with its own connector harness that you splice the existing harness into or is it entirely plug-and play?