Hi all, I have an original 1990s mini city, Which of course came with some incredible speakers but as I've now inherited the car I was planning to put some new speakers in, however with the new stereo I have no idea how to put it in, the old one has a plug in the back (as seen in picture) and I assume I will have to change it or get an adapter or something as this one is from over 15 years ago, so my question is, how do I connect a new stereo system to my car? (The picture is at the back of the stereo slot thing)