2000 Mustang Premium Re-install

I am working on a 2000 Mustang with the premium sound system. I know it’s been covered before but not this particular question. I am removing a system put in by Circuit C*ty: headunit, speakers and amp. They told the previous owner that they had bypassed the factory amp with the new system; the new brand name amp speaker outputs were wired into the clip on harness. Everything worked great.

Well the car has been sold and the previous owner took the newer system out and left the my friend with the factory stuff sitting in the floorboard. We put it all back in and no sound. The headunit powers up but no output. I know the factory wiring for the speakers is good because they left the clip on wiring harness in the car and used the factory speaker wire for the replacement system.

Now my question. Is the premium headunit different from the base? Does the base unit have an internal amp where the premium relies on the external factory amp? I have read up on the 1999 (latest I could find) and know that they came with two separate harnesses for the base and premium systems. But, I can only find one plug for the radio in the dash – unless they stuffed the premium plug further back and I just haven’t located it. Should I get sound from the premium headunit even if it is plugged into the base harness? I would assume yes just greatly reduced but I’m getting nothing. If it matters it also has the separate CD player. Thanks for any help you can give.

Thanks for all the help.  Maybe it was a stupid question, maybe no one has ever re-installed a factory radio in this car but I reconciled the situation.

The 2000's do not have two wiring harnesses.  When guys put the aftermarker unit in they made some mess of a homemade wiring harness - all the wires were the same color.  It was under the seat and I found it this past weekend.  Anyway, the premium unit must be different than the base.  The premium has seven wires coming out of the headunit - ground, constant 12V, and switched 12V.  The rest were not enough to run four speakers.  I remembered the girl telling me that Circuit C*ty had bypassed the factory amp so I started pulling the dash apart looking for another plug for the amp.  I figured the four wires were for the amp and the amp separated those signals into F/R and L/R.  Well the idiot that pulled the aftermarket stuff out shoved the amp's plug way back into the dash and under something that seems to have AC involvement.

Anyway, the base must have at least 11 wires coming from the headunit to run the speakers without the amp, where the premuim only has 7 and a separate plug for the amp input.

The premuim unit has a gray 7 wire plug into the headunit and a black 14 port (as not all ports were used) into the amp.

I assume, the base must plug directly into the black 14 port.