You should put your year and model details in your sig or under your personal details, it would be easier to provide the right link.

Behind the rear seat, there are two access panels. The right one (behind the driver in the UK) has the fuel pump and sender unit. The left one (passenger side) has a second sender unit. There are lines connected to each unit, one for supply and one for return. The lines run up to the fuel filter mounted under the rear seat. I know that the pump is on the right on North American vehicles; I selected a RHD model and it still showed the pump on the right.

See this link for details. I used a 2003, RHD, for this but you can select your correct vehicle details. Look at the tank, vent, and fuel feed sections, all under the Fuel Supply main category.

RealOEM.com * BMW E53 X5 3.0i FUEL TANK/ATTACHING PARTS

The fuel tank does not have an open breather, it is a sealed and pressurized fuel supply system for emissions purposes. Sounds a bit dodgy if fuel is sitting on the sender unit.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue