Wiring color code Woes

Can I post a pic of it showing what I found?

On the vehicle wiring harness I have identified which wires are 12v+ constant and 12v+ switched as well as the speakers. 

On the radio itself, that is where I was having trouble because the colors of the wires do not match any diagram I have found.  How could you test the internal amp to see if it is putting out any signal to the speaker?

When you got this used isuzu radio, did it come with the plug that goes into the back of it?  Or did they just give you the stereo itself without anything to plug into the back of it?

oops sorry I forgot to answer your question about testing the speaker outputs.   First you'd have to find a pair--that is, the positive wire and the negative wire for the same speaker.  Then, with the system turned on you test for voltage by putting the red test lead on one wire and the black test lead on another one.  Positive voltage means the wire touching the red test lead is the positive speaker wire, and negative voltage means the wire touching the black test lead is the positive speaker wire.

Now the only problem with this is that even if it outputs a low-level signal, it will still show up as voltage, just not as much.  It really varies from vehicle to vehicle and stereo to stereo so I don't know of an exact number to look for.

One thing you CAN do that might be easier for you, is to get a test speaker and put that in place of the multimeter leads.  If you can just barely hear sound coming out of the speaker even when it's turned way up, then the head unit is outputting a low-level (un-amplified) signal.  If you can easily hear the sound then you'll know right away that the head unit has a built-in amp and is outputting a high-level (amplified) signal.

Here is a pic of what I have,  I did try what you said with a test speaker and no sound came out....   Chris

For the test speaker thing the only wires that should be connected would be constant power, switched power, ground, and two speaker wires running directly to the positive and negative terminals of an external test speaker.  All of the other wires should be taped off individually, so as not to touch each other or the vehicle's chassis.  The antenna you should plug in cuz you'll need to get radio reception to have sound to test with.  You may have to turn the volume all the way up but even on a head unit with only low-level outputs you should still get sound (albeit very faint) coming out of the test speaker.