The usual Becker Mexico/Europa interface involves a switching stereo jack, and plugging a device into a jack controls whether or the iPod or usual tape/radio audio goes to the preamp/amp. This is not good if you intend to permanently install an MP3 player (maybe in a glove compartment, for example). So, I made a junky PCB for a very tiny DPDT relay, and switched the stereo channels with it. To switch, a wire is brought outside the radio, and grounding this wire performs the switch to iPod audio. Very little current is involved.

Instead of the difficult-to-find (but findable) DIN plug, a conventional stereo 1/8" jack was installed in the back of the radio.

I had done something similar to this in my Europa II's, where a screw mounted to the tuner shaft presses on a microswitch, operating a relay, switching the audio - so that turning the knob (or operating a preset) all the way to the left will activate the iPod audio.

I'm going to install this in my 85 380SL - not period correct and not original, but I could make a case that a digital tuner in a car fundamentally designed in 1970 wasn't period-correct, either. Anyway, I like the way the Beckers look

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86 560SL
With homebrew first gear start!
85 380SL
Daily Driver Project

http://juliepalooza.8m.com/sl/mercedes.htm