I believe you will find X20 pin 24 (hanging off the engine-facing wall of the fusebox) is not occupied and that you "only" have to run the corresponding contact to it, unlock X20, add it and relock X20. I expect, you will pass through the left side firewall and fusebox, one of the more exciting paths through the car.
Do you know the standard trick for feeding wires into the fusebox?
I imagine I could off some other suggestions were I to read through the installation instructions. (I am not sure if you just asked.)