Why does BMW E39 blower fan not work.?

A very common issue on the E39 is the failure of the final stage unit, also called the blower motor resister. This problem will cause the climate control fan to work intermittently, at only one speed, or all the time even when the car is off, thus draining the battery. Replacement part cost is under $100 and is a relatively easy fix to do yourself. Dealers will likely charge an excessive amount to replace this part so it is easier and cheaper to fix yourself. The part is located on the passenger side (drivers for right hand drive) to the left of the glove box, under the carpet that runs along the center console.