Aftermarket stereo & keeping DSP 2001 BMW 540i e39.

I would get the brake bypass module off Amazon as well. It’s about $15

Harness

Scosche VW03B Wire Harness to Connect an Aftermarket Stereo Receiver for Select 2002-Up Audi/BMW/Volkswagen https://www.amazon.com/dp/B0007KPS3C/...

Reverse harness


Metra 71-9003 Reverse Wiring Harness for Select 2002-up BMW 5 Series, Audi, and Volkswagen Vehicles OEM Radio https://www.amazon.com/dp/B0007WQ8VG/...