1 – Understand CAN BUS ENUMERATION in <https://github.com/commaai/hardware/blob/master/harness/v1/open_pinout.sch.pdf>
2 – Before you MITM it, the numbers don’t actually exist—they’re just an abstraction to help the developer understand the direction the signal is coming from. By convention, 2 is the “master,” which sends the signals that control steer, and 0 is the “slave,” which follows those steer commands.
3 – Find at <https://github.com/commaai/hardware/tree/master/harness/v1> the codes for the male and female connectors corresponding to your vehicle and buy in the internet.
4 – Here a a good example of a Hyundai Type – A.
IMPORTANT: Buy the version that already comes with the cables crimped and, most importantly, pre-tested by the seller.
5 – Connect the cables one by one according to the OpenPinout diagram.
6 – If, after connecting your DIY harness to the harness box and properly attaching the male and female connectors, your dashboard lights up with error lights like a Christmas tree, your harness has a bad connection. Take it to an electronics repair shop and explain that some wires have bad contact. Pay a technician to identify and fix the faulty connections.