Brian-bmw is a very informative member here and I would go with his recommendations, period. I have read several of his posts and they are spot on.
Now for my opinion
... I would take it back to the guy and have him remove what he installed; mend back the wires he cut and get the DSP harness back in order. Then I would buy a OEM DSP amp
like this one for $325. Next, install the OEM DSP amp yourself and then take it to the dealer for coding. Depending on what you have in writing with that shop, you may have to settle for a partial refund.
Good luck with all that and I hope you get it back in order!
