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.
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: