This is the wiring loom you need to add the 3.5mm auxiliary input to W203 C and W209 CLK class which have an intermediate connector in the passenger footwell wiring duct.

It is also suitable for W208 CLK and W210 E class cars that have COMAND factory fitted but do not have the AUX connector in the glovebox

It is simple to install as the hole for the AUX connector is pre-drilled in the glovebox and so you just knock out the hole and install the wiring.