stereo wiring diagram on 1990 Mercedes-Benz 420SEL

My son bought a 1990 Merc-Benz 420-SEL -- he is trying to replace the factory radio with an up-to-date CD/radio system -- does anyone know where he can get a diagram of the wiring/speakers etc? Thanks.

Asked by for the 1990 Mercedes-Benz 420SEL
most libraries have an electronic reference called alldata. its gives wiring diagrams.