Since the Curt # C11381 and every other receiver I can find will only fit the sedan versions you are going to have to have a custom receiver fabricated for your 2012 BMW 523i F11 wagon. The Curt Weld-On 2" Receiver # E17 is a great starting point as it provides a large mounting surface in which to weld on cross tubes which can then attach to the chassis.
I recommend installing the 4-Way # C59236 when you have the receiver fabricated. This is the most common connector for small trailers... view full answer...