This is an F25 wiring diagram for the OEM hitch.
skalberti mentioned to me that the fusebox connections are the same, and if you need another diagram to look at, beside the more detailed one that comes with the Invisihitch kit, you may find this useful, especially before you take the plunge.
Document: >
BMW F25 - X3 Hitch Instructions .pdf (in case source removes the document)
Source: >
http://www.bavauto.com/Assets/inst_pages/ins429.pdf