Well, that sucks. I assume you speak their language?

Find out if they know how to work with the balanced preamp HU output in a non-DSP BMW. (Or, since you probably won't get admissions of ignorance, try for "have you done this before? How?)

Then do the unplug tuner, unplug DSP amp, plug in tuner, check for DSP still in menu thing.

If DSP is now gone from the menu, they can interface to the HU output wires as described. If they don't know those wires, they have to find them. If they don't know how to find them, get outta there.

But good luck!

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musicar northwest, portland oregon

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