"Rob Beech" <Mail@robbeech.com> haute in die Tasten:

It definitely is a 3.5 mm stereo plug, which is used for most walkman
headphones. It is there to attach a portable CD player, but a MP3
player also works flawlessly. The 3.5 mm socket has an integrated
switch, so if you plug in a plug while the radio is running, the
device will switch to AUX, which means auxiliary source. If you press
the SRC button, you toggle between Radio and AUX, if you have a
Musicasette inserted, you have three options.

Many mobile players have two output sockets, one to plug in a pair of
headphones, this ofte is marked green, and another which is called
"Line" or "Line out". The green socket is connected with the player's
volume control lever, the Line out socket delivers a fixed signal
strength. When conneting your player to the radio, you need a cable
with one 3,5 mm stereo plug at each end (Radio parts shop). Switch
off both deviches, connect the Line out socket with the sochet at the
radio and then have a test. The volume of the radio should nearly be
the same if you switch from radio to your player and back. If the
player is significantly louder or quieter than the radio, set the
player's volume to minimum, connect the player's headphone socket
with the radio and start increasing the player's volume until it has
reached the level of the radio. When you activate traffic
announcments (TA-button), the radio will switch off the AUX socket
and play traffic announcments, when they are broadcasted. Afterwards
the radio switches back to AUX.

One big drawback is the power supply for the auxiliary player. I have
tried various players and various cigarette lighter adapters, but I
always had annoying electrical interferences through the power
adapter. Does anybody know if it is possible to run the power adapter
with electricity taken directly from the radio wiring. It seems that
this line is somewhat "filtered". Up to now I alwys use batteries to
power my MP3-CD portable.

Frank