Some questions about the new love of my life....

So, in a fit of madness, I decided that my very first car should be a black 2001 P38. The car itself seems sound, it hasn't dropped lost any coolant at all in a week of stop / start driving, the EAS works fine...even the sat nav worked after ejecting / reinserting the CD..... however, I think someone has been tinkering with the audio.

There's no CD changer at all, which I'm not particularly bothered about. The subwoofer doesn't work, and the cable to the sub has another 3 wire cable coming off it marked 'phone: + / - / mute'. I assume this isn't standard as a) I can't find any reference to it by googling and b) it's got loads of electrical tape wrapped round it. Near the rearview mirror there's what looks like a small mic so I assume at some point this has had some sort of phone connection kit in it. Oh, also, behind the panel in the boot, near the sub there's a small blue box that looks suspiciously new.....I have no idea what this is. I'll see if I can grab a picture tomorrow.

Anyway, onto my question..... what's the best way to go about figuring out why the sub isn't doing anything? One of the cones has perished, so I know I need to replace the speakers anyway, but before I fork out for some new ones I'd like to know why nothing at all is coming out of it.

Sorry for the rambling, the multitude of wires in the boot has me intrigued!