Audio Systems: 12-Aux, audio power amplifier, stereo head unit

Question
I am referring to the input power from the vehicle 12 volt red(aux) and yellow(const) wires into the rear of the stereo.  When I plug the stereo into the harness i get the constant 12v (the unit warms up) then when the ignition is on the stereo should turn power up but does not.  I hooked the stereo up to a power supply and began troubleshooting but I can't get very far without a service manual.  Just trying to find a place to get one.  
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Followup To

Question -
Good evening,

I have a JVC KD-LH810 car stereo head unit.  Recently i have had problems with the 12V aux power cutting on and off.  The constant 12 is fine and the unit will power up as if the car was not running.  I have checked car power and its fine.   I have traced the signal into the unit and quickly loose the path shortly after the input.  I was curious if you know of a place to get a schematic of this unit, or if there is a common issue you have heard of similar to this one.  

I've worken in electronic calibrations for 6 years and have a degree so you can be as technical as you need. I just have no experience with car audio.

Answer -
JVC owners manul is not so helpful for this kind of problem. You would need the service manual- which costs money to obtain.

Not sure I understand your question, so let me clarify:

Thid unit has 4 each 20w amplifiers builtin.  Is that what you are using to power the loudspeakers?

And, what is the problem with the aux power cutting off?  Are you saying it kills power to the power amps?  Or are you using a separate power amp?

In any event, if you are running an external amp you can bypass the head unit and put in its own power switch; an inconvenience, but give you manual control over its ON and OFF operaton.

Let me know more - and if I am on the right track.

Cleggsan  

Answer
I am not clear if the problem is a car wiring dilemna or an audio power amplifier that is not working properly.

But, Once again, you should wire a bypass to the power amp to prove it is working when it gets direct power.

A service manual may not help you much in terms of the routing of the wires that get from the car electrical system and ignition to the amp.

Take a look into installdr.com and look for the picture diagrams of how the wiring installation goes for your vehicle. Maybe that will help.