I use a clamp-on DC ammeter.

 

I’ve done parasitic draw tests many times when I was a car audio installer, and I’ve always done it at the battery. I don’t know if it can be done the way you’re describing, but I know it works at the battery. Sometimes it’s more than one circuit that is staying awake or a weird combination of things. I just worry that only testing it at the fusebox will give you misleading results.

 

The Cayenne is an electrical nightmare.

One common fault is the alarm system that randomly triggers the door actuators.

If the vehicle is in a secure location leave it unlocked overnight to see if this impacts the battery voltage.

This manual contains the wiring diagrams for the first generation models.

A dropobox account may be needed to view it.

https://www.dropbox.com/s/z1ot6zvqk6...8.pdf?e=1&dl=0