Plugs behind dash

[Traincliff]

Hi. I have just changed out the original Clarion  radio cassette in my Berlingo based Romahome for the similar radio cd unit. This was a simple job of pulling out the old one and swapping the plugs to the new unit. All done and seems fine. My query is though that I found another smaller plug not used in the radio behind the dash with the radio plugs. It is only about a centimetre cubed in size and a sort of orangey brown in colour. Does anyone know what it is for please? I am just inquisitive to learn what it is for.

Kind regards to all 
Cliff