A mysterious little black box

Hello all... I'm brand-new to my Super Custom G and even newer to this forum - but... from the dashboard hangs a little black box (with no markings) about half the size of a matchbox.  If I could work our how to attach a photo of it all to this message I would do so.  I suspect that it has something to do with the immobiliser that has been fitted but I can't see why it should hang on a wire through the dashboard.  If only I could attach a picture...  oops! I think I've stumbled on the picture option! Let's see...