I am not familiar with anything labeled "Interfire A-800.5." A DSP amp will be manufactured by Lear. Older DSP amps were made by Loewe and Philips. All BMW parts have a part number in the format "xx.xx-x xxx xxx" or just the last 7 digits "x xxx xx." Look for the BMW part number on the part, then we can identify it. Or, post a picture, and maybe we can recognize it.

The second $185 that the mechanic charged you should be refunded. There is absolutely no rewiring that could have been possible. All BMW electronic parts are plug-and-play replaceable-- other than unplugging something and plugging the something back in, a mechanic can not really change anything. Plus, if he coded it the first time, then there is no coding that would have been possible the second time.

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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