Finding wiring diagrams to install 'modern' features like a CD player in a car like a 1983 Mercedes 300d can be tricky, but there are places online you can find out more.
A fellow Mercedes driver who had a similar problem posted an appeal at benzworld.org, so try there first.
Other helpful sites include mercedes-benz-manual.blogspot.com, where you can download a copy of the electrical wiring diagrams, included in the driver's repairs manual. You can also find an extensive range of Mercedes wiring diagrams at wirediagram.com
For specific assistance relating to the installation of CD players, and the parts required, try tune-town.com, peachparts.com or geekparts.com.
Failing all this, try the Mercedes-Benz users support site at mercedes-benz.custhelp.com or call on 1-800-367-6372 option 4.
Lots of YouTube users post videos on their site regarding car repairs, and might also be able to assist. Try watching this film and leaving a message in the comments field: