you will need a wiring diagram, that can be found by searching this forum. Then, with your aftermarket HU, connect the wires following the diagram and the label on the aftermarket connector. It will be included with the radio. You could buy a factory harness, but since the wire are already cut, its up to you. thr other plug is the antenna input. you will need an adapter for it, available usually at any car audio location.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>