I would check the connections in the 14 pin connector in the far rear of the drivers side - back by the taillight but inside the engine compartment. The male and female parts can get pushed out a little and you can't tell from looking at the outside of the connector.
Check the connector to the CD box - the individual connectors can slide out.
Here is a link to an SC wiring diagram. Diagram 1 Page 2 has the CD box. If you take some time and use a multimeter to check for continuity you can chase down the connections one at a time. It is something simple. Finding it takes time.
Pelican Parts - Porsche 911 Parts Listings & Diagrams