i looked it up. there are a couple of good diy's and all you need is a tiny bit of electrical knowledge and a wiring diagram for the bose harness. i did the install for mine but i havent gotten my amp yet. (dhl FTL). the install was painless, but i have all the easy strippers and heatshrinking stuff cause im an EE student. the low to high level converter is like 20 buck on ebay.
edit: it might be high to low not low to high. im not sure. the input is a splice to the existing rear speaker inputs to the bose amp and the output is RCA for your new amp.
hope this helps!
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2001 Audi S4