Only thing I can think of so far for an output is the power antenna wire of the tuner. That's a fair waste though and even then it's speed of switching mightn't be good enough. You really need a direct IO from the CPU for this to work properly I would say.

I think the tuner interface is the best option for a possible digital output. As I don't intend to remove the factory headunit, I won't require a tuner anyway.
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Christian
#40104192 120Gb (no longer in my E36 M3, won't fit the E46 M3)