what you saw behind the carpet was the factory amp. The CD plug is on the other side. The plug and powercord are usually ziptied together at some point, so following one will lead you to the other.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>