Car Stereos: Speakers stop working when car turns on, 2003 honda crv, internal amplifier

Question
I was wondering if you could help me with a problem I'm having in my 2003 Honda CRV. A couple months ago I installed Polk DB651's in all 4 doors and they worked great (A little bit of modification due to size, but no problems). Then one day the driver and front passenger door speakers stopped working. Well today  I finally had the time to fix the problem and for the most part I did. I just didn't have something plugged in tight enough so it had slipped off. Anyways, all 4 speakers worked fine until I started my car. The speakers ran for about 5 seconds and then just stopped playing music.  I have a JVC KD-HDR50 head unit, but thats the only other after market audio piece in my car. This had never happened before. If I turn off the car, take out the keys, and try it again, it does the same exact thing. It can play as long as it can if the car is not running. Also, the head unit stays on and is fully functioning, it also seems like the speakers are fine, it's just no music being transfered from the head unit to the speakers. I checked fuses before I started dissecting my car but the are fine it seems. Also I barely touched wires, just the harness wires that lead to the speakers through my door.  Any clue what the problem could possibly be?

Thank you so much for any help

Answer
Hi Bo,

Sorry about the delay in answering this question.  Hopefully this arrives in time to do you some good.

Generally, when you have a head unit that loses all audio output, but keeps the display working like normal, then it's likely to be a speaker wiring problem.  What makes yours unusual is the fact that it's only happening when the engine's running.  However, I'd probably troubleshoot the problem the same way:  use a multimeter to test the speaker wires for a short to chassis ground.

Just about all after-market head units have a built-in protective circuit that will cut off the internal amplifier if there's a wiring fault.  This means that a problem with a single speaker wire can kill the sound to all the speakers.  The easiest way to track it down is to use a multimeter, set for continuity function, and look for a speaker wire that shows continuity with chassis metal--ie, a metal part of the dash structure.  If you find this happening, you can check that speaker directly and see if there's a wire that's pinched or touching metal at the speaker location.  

The fact that it's only happening after you start the car means it may be an intermittent contact; possibly the vibration of the running engine is triggering the problem.

Hope this helps!

Brian