amp not always powering on

Newbie to this site.

I have a 2007 Ford Mustang GT with the shaker 1000 stereo system.  This is the unit with two factory 10 inch subs and corresponding amps in the trunk.  I did not like the sound of the factory speakers and replaced all 4.  I added an Infinity amp (2 channel amp) to the two rear speakers and this is when my problems started.

My problem is that although there is a good ground and power supply, the amp for the rear speakers will not always power on, thus the rear speakers will not have sound.  Turning the radio on and off many times and/or turning the key on and off a few times with the radio on will usually get the amp to power on and the speakers to work fine.   If I unhook the amp and run the speakers from dech/head unit power only they work fine so I can rule out a problem with the radio or speakers.  Once the amp powers up the radio can be turned on and off repeatedly without any problems, however after the car has been shut off for more than a couple of hours the same thing happens all over again.

Is it possible that since we tied in to the remote turn on that powers the factory subs at the rear of the vehicle there is not a steady/proper amount of voltage getting to the Infinity amp to power it up?  If so, should we try and run the remote amp wire to the back of the head unit rather than how we tied it in to wiring for the factory subs?  If so, is there some type of remote turn on wire at the back of the deck that would work better?   I am not sure if I have a bad amp that I need to send back to Crutchfield or if the source where we have the "power on" wire running is incorrect and the problem is a wrong install on my part.

Thank you for your help with this puzzling problem

For the record this problem began the first day the amp was installed (2 weeks ago) and has gotten no worse nor any better.

You have two concerns here but are addressing only one.  Yes, the amp may not be getting a B+ (~12volt) signal to turn it on.  The other factor missing here is how you have the inputs to the amplifier.  For troubleshooting, first try to take the B+ from another source withing the car.  You should probably find a switched 12 volt source near the amp's location, which you can tap for power.  In doing so, the amp will power up when you start the car.  Finding a switched hot wire amounts to testing with a meter.  Always be sure to remove the battery neg cable before cutting through the insulation of any wire.  It can thus be a tedious method but you'll eventually find a wire that will power up the amp.

The other factor, the input:  ensure that the amp is actually not powering up by looking at it when the deck is turned on.  It is required, if you are using the factory deck that has no line level output, that the input signal is fed into the amp via the high level tapped into the speaker wires.  Often, these amplifier high level inputs don't work correctly with every car.  A high quality aftermarket LOC is very often needed, as these allow for adjustment of input voltages. 

This is one source for various adapters that enable adding to factory sound systems:  http://www.davidnavone.com/adaptor_products.htm

Others:  PAC, Peripheral, et al.

Thanks Stevdart

I will research and get back with you by the weekend with our findings.

Jeff

Ok-  you were exactly right mike12volt.  The factory amp for the sub where we tied in initially for the remote turn on measured only 5 volts, so we ran a lead to an accessory power wire that had 12volts and all is well.  There was not a remote turn on wire at the back of the HU like I thought there would be, thus we had to locate another 12 volt source.

It finally works like it should.

Thanks for everyone's help.

Jeff