1. Ideally, you should bypass the fader. This link: w126 stereo fader shows how. Even though it's for a different model car, the wiring is the same.

2. Which radio do you have exactly? Perhaps some photos?

There will be one plug which has four connectors, one connector is offset. This whole plug is the power plug, containing the constant, illumination, ignition and ground connectors.

The other plug (with brown wires) could be the Speed-Dependent Volume Control plug, which isn't utilized by the Sony radio.

The connector with the metal clip could be another ground connector.