HoustonMtn, M3Armand:

The E70 is technically pre-wired for a brake controller.

The problem is that the connector is in the rear compartment in the panel on the right side, and the brake controller needs to be where the driver can reach it (front left). That means I had to run a wire bundle from under the steering wheel (where I put my controller) to the connector in the panel in the back.

This is an improvement over the E53, where, I'm told, one has to run wiring down to the trailer connector itself, which is more work.

An update on my situation: I've towed our 16' Airstream (3,500 Lb. GVWR) a couple of times now from San Francisco to Reno (over the Sierras) using the weight distributing hitch that I got with the trailer, and it's great. I've never noticed any sway, and I've never felt like the X5 was working very hard to pull the trailer. Granted, it's not a very heavy trailer, but it's easy to forget it's back there until you look in the mirror.