Also the part number from the cable housing (possible the near by cables) plus the exact wire colour codes always help searching from wiring diagrams etc. if this isn't obvious for someone.
Im sorry dident expalin where it is. Its attached with the stereo cables. I cant se where it should fit on back of the stereo. Its an Mercedes C-Class forme 2006. With Audio 20 installed froem factory
Chances are it's for an option or unit you don't have fitted? If your stereo is working OK then nothing to worry about. I'm sure one of our ICE experts will have the answer.
Again, the wire colours would help. It isn't easy to see the colour coding from the photo. If the wires are brown/yellow and red/yellow, it would be for an external fan. Does your head unit actually have a fan, it appears for Audio 30 at least. The unit would work without the fan wires connected as long as it does not get too hot. But Audio 20 may not have a fan at all and then the wires are there waiting for another head unit.