This is the Focal IW MBZ Y-ISO Mercedes-Benz Amplifier Harness.

The Focal Inside IW MBZ Y-ISO HARNESS car radio adapter cable is designed for the IMPULSE 4.320 four-channel digital amplifier. One meter long, it connects to the original Mercedes-Benz car radio and allows you to connect Focal Inside extension cables quickly and easily. This cable is designed specifically for Mercedes-Benz vehicles (E-Class from 2016, S-Class from 2013, C-Class from 2014, until 2019). Connection to the extension cable is via ISO connectors; no soldering is required.