I recently watched a great episode of Tricked Out where a MINI was worked on to improve the stereo system.
The actual work done on the MINI was nowhere near extreme – they just added an amplifier and subwoofer to the car, leaving the factory head unit and speakers. But the program goes into excellent detail on the work involved.
If you visit the Tricked Out website, you will find a detailed step-by-step photo sequence showing the removal of the MINIs head unit, connecting wiring adaptors and line level converters, running wires to the trunk and connecting to an amplifier, and finally installing the subwoofer.
The subwoofer is particularly interesting since it is custom shaped for the MINI and uses space behind one of the “cubby” storage spaces in the trunk to give extra resonance space with minimal intrusion into the trunk.
I could not find any reference on the Tricked Out website, but a quick bit of Googling found me this page which looks to be the same enclosure …

If you did not see the Tricked Out episode, and prefer a video guide to removing the MINI head unit, here’s a video that I captured from the broadcast: