In order to have your AutoHold retrofit, you will need to buy Auto Hold switch [4M1927143] and wiring harness. I bought my switch from bm-parts.nl and the harness from Aliexpress. You can also make your own harness, of course.
Once you have your parts ready, you can start running wires and connecting.
Your AH switch [E540] has 4 pins which needs to connect to;
1- Front Electronics Unit [J519] : B-31
2- Handbrake button [E540] : PIN-6 (GROUND)
3- Front Electronics Unit : [J519] A-17
4- Handbrake button [E540] : PIN-5 (+)
Coding is a bit tricky. Use VCDS to code like below;
– Module 9 Byte 15 Bit 6 is set from 0 to 1
– Module 3 Byte 18 Bit 6 is set from 0 to 1
– Module 3 Byte 30 Bit 1 is set from 0 to 1
If your car doesn’t have Adaptive Cruise Control, these will work just fine. With ACC, I used ODIS-E to do the coding, since VCDS cannot calculate binary and update checksum correctly. If you are new to ODIS-E, you will need to have au49 workspace and its files in order to code properly.