This is the Axxess AXVI-1785 Mercedes Benz Head Unit Interface.

Provides accessory power (12volt/10amp)
retains RAP (retained accessory power)
Used in non-amplified systems or when bypassing amplified systems
Provides NAV outputs (Parking brake reverse mute V.S.S.)
High level speaker input 

Applications

• Mercedes C Class 2001-2004
• Mercedes CLK Class 2004
• Mercedes CLK500 2003
• Mercedes CLK55AMG 2003