I have not read of anyone who does DSP amp repairs. The best bet is to buy a used one from eBay. Just look for one of the newer models made by Lear. After you get it installed, you will need to get it coded unless you know for certain that it cam to a car just like yours (same model and A/V/nav features).

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2000 E38 750iL Highline Edition, Upgraded with the lastest 2006 X5 Electronics
MKIV Navigation, 16:9 Display, Bluetooth, TV/Video with Backup Camera, Sirius, and Aux Input