There should not be any connector cut, otherwise the DA2 would not have been working at all. It requires the MOST connector as is. In the worst case it may be missing a MOST jacket which you can get at the dealer.
Unplug DA2 and connect OEM amp back. The dealer is correct.
-2015 F30 320i, Imperial/Oyster, 8AT, ED
-2012 E92 M3 Jerez Black/Rust Brown, M-DCT, ED: Gone
-2008 E90 M3 Melbourne Red/Bamboo, 6MT: Gone
-2007 MK5 GTI United Grey/Grey, DSG: Gone
-2004 E46 M3 Carbon Black/Cinnamon, 6MT: Gone